Aerogels, calcined and crystalline articles and methods of making the same

ABSTRACT

Aerogel, calcined articles, and crystalline articles comprising ZrO 2 . Exemplary uses of the crystalline metal oxide articles include dental articles (e.g., restoratives, replacements, inlays, onlays, veneers, full and partial crowns, bridges, implants, implant abutments, copings, anterior fillings, posterior fillings, and cavity liner, and bridge frameworks) and orthodontic appliances (e.g., brackets, buccal tubes, cleats, and buttons).

BACKGROUND

Dental restorations such as crowns and bridges are commonly made by whatis known as the porcelain fused to metal process. A metal coping orsupport structure is covered with layers of glass having differentlevels of translucency. Opaque layers cover the metal to hide its colorfollowed by more translucent layers to improve the aesthetic appearance.In recent years there has been a trend toward all ceramic dentalrestorations; crowns, bridges, inlays, etc. In particular, metal copingswhich provide the structural support for crowns and bridges are beingreplaced by high strength ceramics. These materials have color andtranslucency characteristics which better match the natural tooth andproduce a more aesthetic appearance.

Zirconia is a preferred material for this application because of itshigh strength and toughness. Pure zirconia exists in three crystallineforms; monoclinic, tetragonal, and cubic. Monoclinic is stable from roomtemperature up to about 950-1200° C., tetragonal is the stable form from1200° C. to about 2370° C., and cubic is stable above 2370° C. Sinteringzirconia to high density generally requires temperatures above 1100° C.The monoclinic phase typically transforms to tetragonal duringsintering, but then transforms back to monoclinic on cooling.Unfortunately, this transformation is accompanied by a volume expansionwhich causes the ceramic to crack and usually break apart. Stabilizingagents such as yttria can be added to zirconia to avoid this destructivetransformation. Typically, when greater than about 2 mole percent yttriais added, the tetragonal phase can be retained as a metastable phaseduring cooling. When more than about 8 mole percent yttria is added thecubic phase forms at sintering temperatures and is retained duringcooling. Between these levels of yttria a mixture of the tetragonal andcubic phases are formed during sintering and usually retained duringcooling. Under rapid cooling conditions the cubic phase may be distortedto form another tetragonal phase known as tetragonal prime.

Zirconia stabilized with 2-3 mole percent of yttria is especiallyattractive as a structural ceramic because it can exhibit a large degreeof transformation toughening. At this level of yttria the materialconsists largely of metastable tetragonal crystals with the balancebeing cubic or tetragonal prime. When a crack passes through thematerial it triggers transformation of the tetragonal crystals near thecrack tip to the monoclinic form along with the associated volumeexpansion. This localized expansion resists the extension of the crackacting as a toughening mechanism.

The amount of toughening is dependent on the grain size, yttria content,and the matrix constraint. As the grain size is reduced the tetragonalform becomes more stable. Optimum toughening is obtained when the grainsize is just below the critical grain size where the tetragonal phase ismetastable. If the grain size exceeds the critical size the tetragonalphase can convert spontaneously to the monoclinic form throughout thebulk of the material causing widespread cracking. If the grain size istoo far below the critical size than the tetragonal crystals are sostable that they will not revert to monoclinic in the stress field of acrack tip. As the amount of yttria stabilizer in the tetragonal form isreduced the tetragonal form becomes thermodynamically less stable andthe critical grain size is reduced.

Generally as discussed in Scripta Materialia, 34(5) 809-814 (1996), atan overall composition of 3 mole percent yttria excellent toughening isobtained with grain sizes near 500 nm, but the toughness is reduced atgrain sizes near 100 nm. While the overall composition is 3 mole percentyttria, the tetragonal phase contains about 2 mole percent yttria, theremainder is in the cubic phase that is also present. As the bulk yttriacontent changes from 2-9 mole percent over the range where thetetragonal and cubic phases coexist, the yttria constant of thetetragonal phase itself is constant. As a result the critical grain sizeis also constant over this range. It should be expected then that as thegrain size of the two phase materials is reduced to values approaching100 nm the effect of transformation toughening will also be reduced.Some insight into the absolute minimum grain size which can provide atoughening effect can be found in studies of pure tetragonal materialswhere the amount of yttria can be reduced to lower levels. Further asgenerally discussed in Journal of Acta Materialia, 50, pages 4555-62,(2002), if the amount of yttria is reduced to 1 mole percent, excellenttoughening can be obtained at 90 nm, but falls rapidly below about 75nm.

As the overall yttria composition increases over the tetragonal pluscubic range there will be decreasing amounts of the tetragonal phasepresent. Therefore, the toughness and strength of materials would beexpected to drop as the amount of tetragonal phase in the material isreduced.

Matrix constraint is the resistance adjacent crystals exert on atetragonal crystal as it tries to transform (expand) against itssurroundings. In a fully dense material the adjacent grains provide ahigh degree of matrix restraint. A porous material provides room forlocal expansion and therefore less matrix restraint.

In summary, optimum toughening and strength are expected when the grainsize is just below the critical grain size for a given yttria content,the material is fully dense, and contains a high fraction of thetetragonal phase. Improvements in optical translucency achieved by grainsize reduction must be balanced against the loss in toughness expectedat grain sizes below 100 nm, and especially below 75 nm. Improvements inoptical transmission which might be expected with higher cubic contentmust also be balanced by the loss in toughness expected with fewertetragonal grains.

The high strength and toughness of zirconia makes milling of intricateshapes from fully dense material very difficult. The milling operationis slow and tool wear is high. To overcome this limitation the zirconiamay be milled to shape using a partially densified (calcined) body,referred to as a mill block. The mill block is typically 50% dense. Ithas sufficient strength for handling and is readily milled with minimaltool wear. The shaped restoration can then be heated (sintered) to forma fully dense article which is strong and somewhat translucent. Duringthe sintering process the material shrinks roughly 20% in lineardimensions as it becomes denser. This shrinkage can be accounted for byusing optical scanners and computer design to obtain a three-dimensionalimage of the restoration. This image file can be expanded to compensatefor the sintering shrinkage, then transferred to a computer controlledmilling machine to produce the restoration. Sintering at hightemperature produces the final densified restoration.

While zirconia has a limited amount of translucency, higher translucencyis desired to achieve even better appearance for dental applications.Ceramics are often opaque in appearance due to the scattering of lightby pores in the ceramic. In order to achieve even a limited level oftranslucency, the density of the ceramic is typically greater than 99%of theoretical. Higher clarify can require levels above 99.9% or even99.99%. Two methods known in the art for achieving very high densitiesin ceramic materials are hot isostatic pressing and spark plasmasintering. However the equipment required for these methods isrelatively expensive and is not well suited for use in most dentalrestoration laboratories. Also, protective atmospheres and/or graphitedies used in this equipment can lead to discoloration of the zirconia bychemical reduction (loss of oxygen from the zirconia).

Another factor which can limit the translucency of ceramics is thepresence of two or more solid phases having a different refractiveindex. In such cases to improve transparency, it is necessary to reducethe size of these phases well below the wavelength of visible light toavoid excessive scattering. Even in single phase materials scatteringcan occur if the material exhibits birefringence (i.e., has a differentrefractive index in different crystal directions). Light is thenrefracted and reflected (scattered) as it crosses grain boundaries fromone crystal to another having a different orientation. In this case thecrystallite size needs also to be less than the wavelength of visiblelight to achieve high levels of translucency. For these reasons highlytranslucent ceramics are often fabricated from single phase, cubicmaterials which exhibit no birefringence. In the case of zirconiaceramics, however, strength is compromised as the cubic form of zirconiais not transformation toughened.

Sintering of nanoparticles (10-100 nm) is one way to produce smallgrains in ceramics. The small size increases the driving force fordensification (i.e., the reduction in surface area). Unfortunatelynanoparticles tend to form strong agglomerates which do not easily breakdown during pressing operations. The particles within an agglomerate aregenerally packed more densely than the surrounding particles leading toa non-uniform pore structure in the final sintered body. Obtaining fullydense, highly translucent articles, without the use of high pressuretechniques has proven difficult.

Sol-gel processing of nanometer sized particles is one way of avoidingagglomeration and achieving the high density and small grain sizedesired for both strength and translucency. The difficulty with thisprocessing approach is that it does not lend itself to the production ofrelatively large articles. It has been successfully applied to themanufacture of fibers, beads, and abrasive grit, but it is not wellsuited to the production of bulk articles greater than about 1 mm insize. The problem has been in drying the gel where capillary forces leadto high shrinkage and cracking unless relatively lengthy dryingtechniques are used. In addition, for nanoparticle sols having organicstabilizing agents to keep the particles well dispersed, it can bedifficult to remove these organics during heating without crackformation. The dense packing of nanoparticles in the dry bodies meansthe open pore channels needed to remove volatiles are relatively smallleading to pressure build-up within the body. Also, if the organicsseparate the individual particles of the dry body, shrinkage will occuras the organics are removed. Since organic is most easily volatilizednear the surface, non-uniform shrinkage is likely.

It is known that supercritical extraction of liquid from bulk gels caneliminate cracking during drying because there are no capillary forcespresent. Further, the lack of capillary forces to pull the particlestogether tends to lead to more relatively open structures commonlyreferred to as aerogels. Aerogels can have pore volumes of 90% or more.The more open structure of an aerogel would be expected to aid inuniform volatilization of any organics present. However, the lowrelative density of an aerogel (typically <10% of theoretical) presentsa problem as it is generally known that high packing densities of theparticles are desirable for densification during sintering. While silicaaerogels have been successfully sintered to full density, it has notbeen considered possible to sinter crystalline aerogels to full density.Silica sinters by a viscous flow process which is much faster than thesolid state diffusion mechanisms responsible for sintering crystallinesolids.

SUMMARY

In one aspect, the present disclosure describes an aerogel (in someembodiments, a monolithic aerogel (i.e., having x, y, and z dimensionsof at least 1 mm (in some embodiments, at least 1.5 mm, 2 mm, 3 mm, 4mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, or even at least 10 mm)) comprisingorganic material and crystalline metal oxide particles, wherein thecrystalline metal oxide particles are in a range from 3 to 20 volumepercent, based on the total volume of the aerogel, wherein at least 70(in some embodiments, at least 75, 80, 85, 90, 95, 96, 97, 98, or evenat least 99; in a range from 70 to 99, 75 to 99, 80 to 99, or even 85 to99) mole percent of the crystalline metal oxide is ZrO₂. An advantage ofembodiments of aerogels described herein is that they can be sintered toa fully dense material despite the fact that they have low relativedensity and are comprised of crystalline particles.

In another aspect, the present disclosure provides a method of makingaerogels described herein, the method comprising:

-   -   providing a first zirconia sol comprising crystalline metal        oxide particles having an average primary particle size of not        greater than 50 nanometers, wherein at least 70 mole percent of        the crystalline metal oxide is ZrO₂;    -   optionally concentrating the first zirconia sol to provide a        concentrated zirconia sol;    -   adding a radically reactive surface modifier to the zirconia sol        (or understood to be the concentrated zirconia sol, as        applicable) to provide a radically polymerizable        surface-modified zirconia sol;    -   adding a radical initiator to the radically polymerizable        surface-modified zirconia sol;    -   heating at at least one temperature for a time sufficient to        polymerize the radically surface-modified zirconia sol        comprising the radical initiator to form a gel;    -   optionally removing water from the gel via alcohol exchange to        provide an at least partially de-watered gel; and    -   extracting alcohol from the gel (or understood to be the at        least partially de-watered gel, as applicable) via super        critical extraction to provide the aerogel.

In another aspect, the present disclosure provides a crack-free,calcined metal oxide article (e.g., having x, y, and z dimensions of atleast 5 mm) a density in a range from 30 to 95 percent of theoreticaldensity, and an average connected pore size in a range from 10 nm to 100nm, wherein at least 70 mole percent of the metal oxide is crystallineZrO₂, and wherein the crystalline ZrO₂ has an average grain size lessthan 100 nm.

In another aspect, the present disclosure provides a method of makingcrack-free, calcined metal oxide articles described herein, the methodcomprising heating aerogels described herein for a time and at at leastone temperature sufficient to provide the crack-free, calcined metaloxide articles.

In another aspect, the present invention provides a crack-free,crystalline metal oxide article having x, y, and z dimensions of atleast 3 mm and a density of at least 98.5 (in some embodiments, 99,99.5, 99.9, or even at least 99.99) percent of theoretical density,wherein at least 70 mole percent of the crystalline metal oxide, isZrO₂, wherein from 1 to 15 mole percent (in some embodiments 1 to 9 molepercent) of the crystalline metal oxide is Y₂O₃, and wherein the ZrO₂has an average grain size in a range from 75 nanometers to 400nanometers. In calculating the theoretical density, the volume of unitcell is measured by XRD for each composition or calculated via ionicradii and crystal type.

$\rho_{theory} = \frac{N_{c}A}{V_{c}N_{A}}$

Where

N_(c)=number of atoms in unit cell;A=Atomic Weight [kg mol⁻¹];V_(c)=Volume of unit cell [m³]; andN_(A)=Avogadro's number [atoms mol⁻¹].

In another aspect, the present disclosure provides a method of making acrack-free, crystalline metal oxide article having x, y, and zdimensions of at least 3 mm and a density of at least 98.5 (in someembodiments, 99, 99.5, 99.9, or even at least 99.99) percent oftheoretical density, wherein at least 70 mole percent of the metal oxideis crystalline ZrO₂, wherein from 1 to 5 mole percent (in someembodiments 3.5 to 4.5 mole percent) of the crystalline metal oxide isY₂O₃, and wherein the crystalline ZrO₂ has an average grain size in arange from 75 nanometers to 175 nanometers, the method comprisingheating a crack-free, calcined metal oxide article described herein fora time and at at least one temperature sufficient to provide thecrack-free, crystalline metal oxide article.

In another aspect, the present invention provides a crack-free,crystalline metal oxide article having x, y, and z dimensions of atleast 3 mm and a density of at least 98.5 (in some embodiments, 99,99.5, 99.9, or even at least 99.99) percent of theoretical density,wherein at least 70 mole percent of the crystalline metal oxide is ZrO₂,wherein from 1 to 5 mole percent (in some embodiments, 3.5 to 4.5 molepercent) of the crystalline metal oxide is Y₂O₃, and wherein the ZrO₂has an average grain size in a range from 75 nanometers to 175nanometers (in some embodiments, in a range from 100 nanometers to 165nanometers).

In another aspect, the present disclosure provides a method of making acrack-free, crystalline metal oxide article having x, y, and zdimensions of at least 3 mm and a density of at least 98.5 (in someembodiments, 99, 99.5, 99.9, or even at least 99.99) percent oftheoretical density, wherein at least 70 mole percent of the metal oxideis crystalline ZrO₂, wherein from 1 to 15 mole percent (in someembodiments 1 to 9 mole percent) of the crystalline metal oxide is Y₂O₃,and wherein the crystalline ZrO₂ has an average grain size in a rangefrom 75 nanometers to 400 nanometers, the method comprising heating acrack-free, calcined metal oxide article described herein for a time andat at least one temperature sufficient to provide the crack-free,crystalline metal oxide article.

In another aspect, the present invention provides a crack-free,crystalline metal oxide article having x, y, and z dimensions of atleast 3 mm and a density of at least 98.5 (in some embodiments, 99,99.5, 99.9, or even at least 99.99) percent of theoretical density,wherein at least 70 mole percent of the crystalline metal oxide is ZrO₂,wherein in range from 6 to 9 mole percent (in some embodiments 7 to 8mole percent) of the crystalline metal oxide is Y₂O₃, and wherein theZrO₂ has an average grain size in a range from 100 nanometers to 400nanometers (in some embodiments, in a range from 200 nanometers to 300nanometers).

In another aspect, the present disclosure provides a method of making acrack-free, crystalline metal oxide article having x, y, and zdimensions of at least 3 mm and a density of at least 98.5 (in someembodiments, 99, 99.5, 99.9, or even at least 99.99) percent oftheoretical density, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂, wherein in range from 6 to 9 mole percent (in someembodiments 7 to 8 mole percent) of the crystalline metal oxide is Y₂O₃,and wherein the ZrO₂ has an average grain size in a range from 100nanometers to 400 nanometers, the method comprising heating acrack-free, calcined metal oxide article described herein for a time andat at least one temperature sufficient to provide the crack-free,crystalline metal oxide article.

In another aspect, the present disclosure provides a method of making acrack-free, crystalline metal oxide article having x, y, and zdimensions of at least 3 mm and a density of at least 98.5 (in someembodiments, 99, 99.5, 99.9, or even at least 99.99) percent oftheoretical density, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂, and wherein the ZrO₂ has an average grain size lessthan 300 nanometers, the method comprising pressureless heating in air acrack-free, calcined metal oxide article having x, y, and z dimensionsof at least 3 mm, a density in a range from 30 to 95 percent oftheoretical density, wherein at least 70 mole percent of the metal oxideis crystalline ZrO₂, and wherein the crystalline ZrO₂ has an averagegrain size less than 100 nm for a time and at at least one temperaturesufficient to provide the crack-free, crystalline metal oxide article,wherein the method is conducted at no greater than 1400° C. (in someembodiments, at at least one temperature in a range from 1000° C. to1400° C., 1000° C. to 1350° C., or even 1200° C. to 1300° C.).

In this application:

“aggregation” refers to a strong association of two or more primaryparticles. For example, the primary particles may be chemically bound toone another. The breakdown of aggregates into smaller particles (e.g.,primary particles) is generally difficult to achieve.

“aerogel” refers to a three-dimensional low density (i.e., less than 20%of theoretical density) solid. Aerogels are typically formed from a gelby solvent removal, for example, under supercritical conditions. Duringthis process the network does not substantially shrink and a highlyporous, homogeneous, low-density material could be obtained.

“agglomeration” refers to a weak association of two or more primaryparticles. For example, the primary particles may be held together bycharge or polarity. The breakdown of agglomerates into smaller particles(e.g., primary particles) is less difficult than the breakdown ofaggregates into smaller particles.

“associated” refers to a grouping of two or more primary particles thatare aggregated and/or agglomerated. Similarly, the term “non-associated”refers to two or more primary particles that are free or substantiallyfree from aggregation and/or agglomeration.

“calcining” refers to a process of heating solid material to drive offat least 90 percent by weight of volatile chemically bond components(e.g., organic components) (vs., for example, drying, in whichphysically bonded water is driven off by heating). Calcining istypically done at a temperature below a temperature needed to conduct apre-sintering step.

“crack-free” means no cracks are visible from 15 cm (6 inches) away whenviewed with 20-20 vision (if desired, a microscope can be used whereinthe sample is observed using polarized light in transmission);

“crack” means a material segregation or partitioning (i.e. defect),wherein the ratio of the segregation or partitioning is about 1:10 intwo dimensions, wherein for the thermal untreated material one dimensionunit is above about 40 μm. A surface defect having one maximum dimensionbelow 40 μm is not regarded as a crack.

“ceramic” means an inorganic non-metallic material that is produced byapplication of heat. Ceramics are usually hard, porous and brittle and,in contrast to glasses or glass ceramics, display an essentially purelycrystalline structure.

“crystalline” means a solid composed of atoms arranged in a patternperiodic in three dimensions (i.e., has long range crystal structure asdetermined by X-ray diffraction).

“dental mill block” refers to a solid block (three-dimensional article)of material from which a dental article, dental workpiece, dentalsupport structure or dental restoration can be machined. A dental millblank may have a size of about 20 mm to about 30 mm in two dimensions,for example, may have a diameter in that range, and may be of a certainlength in a third dimension. A blank for making a single crown may havea length of about 15 mm to about 3.0 mm, and a blank for making bridgesmay have a length of about 40 mm to about 80 mm. A typical size of ablank as it is used for making a single crown has a diameter of about 24mm and a length of about 19 mm. Further, a typical size of a blank as itis used for making bridges has a diameter of about 24 mm and a length ofabout 58 mm. Besides the above mentioned dimensions, a dental mill blankmay also have the shape of a cube, a cylinder or a cuboid. Larger millblanks may be advantageous if more than one crown or bridge should bemanufactured out of one blank. For these cases, the diameter or lengthof a cylindric or cuboid shaped mill blank may be in a range of about100 to about 200 mm, with a thickness being in the range of about 0.10to about 30 mm.

“dental ceramic article” means any article which can or is to be used inthe dental or orthodontic field, especially for producing of or asdental restoration, a tooth model and parts thereof.

Examples of dental articles include crowns (including monolithiccrowns), bridges, inlays, onlays, veneers, facings, copings, crown andbridged framework, implants, abutments, orthodontic appliances (e.g.brackets, buccal tubes, cleats and buttons) and parts thereof. Thesurface of a tooth is considered not to be a dental article.

“hydrothermal” refers to a method of heating an aqueous medium to atemperature above the normal boiling point of the aqueous medium at apressure that is equal to or greater than the pressure required toprevent boiling of the aqueous medium.

“in the range” includes the endpoints of the range and all numbersbetween the endpoints. For example, in the range from 1 to 10 includesthe numbers 1 and 10 as well as all numbers between 1 and 10.

“organic matrix” refers to any organic compound or mixture of suchcompounds. The organic matrix often includes one or more organicsolvents, one or more monomers, one or more oligomers, one or morepolymeric materials, or a combination thereof. In many embodiments, theorganic matrix is an organic solvent and a polymerizable composition, ora polymerized composition.

“primary particle size” refers to the size of a non-associated singlecrystal zirconia particle. X-ray Diffraction (XRD) is typically used tomeasure the primary particle size using the techniques described herein.

“sol” refers to a continuous liquid phase containing discrete particleshaving sizes in a range from 1 nm to 100 nm.

“stable” in reference to a sol means that no more than 5 weight percentof the particles within the sol precipitate when the sol is stored forat least one week at room temperature (e.g., 20° C. to 25° C.). Forexample, less than 5 weight percent, less than 4 weight percent, lessthan 3 weight percent, less than 2 weight percent, less than 1 weightpercent, or less than 0.5 weight percent of the particles within the solprecipitate under these storage conditions.

“diafiltration” is a technique that uses ultrafiltration membranes tocompletely remove, replace, or lower the concentration of salts or,solvents from solutions containing organic molecules. The processselectively utilizes permeable (porous) membrane filters to separate thecomponents of solutions and suspensions based on their molecular size.

It is known in the art that when conventional YTZP (yttria stabilizedtetragonal) zirconia is in contact with water (including biologicalfluids containing water) for extended periods of time (which effects canbe accelerated using the Hydrolytic Stability Test in the Examplessection, below) the phase composition at the surface of the YTZP canchange (sometimes referred to as “low temperature degradation”). Thatis, conventional YTZP does not have good hydrolytic stability. Thetetragonal phase transforms partly into to the monoclinic phase whichcan be accompanied with a roughening of the material surface. Forbiomedical applications, for example, a zirconia material is desired toexhibit little or no tetragonal to monoclinic transformation under humidconditions (respectively hydrothermal treatment). Further details can befound, for example, in J. Chevalier, L. Gremillard, S. Deville, Annu.Rev. Mater. Res. 2007, 37, 1-32 and J. Chevalier, L. Gremillard, A.Virkar, D. R. Clarke, J. Am. Ceram. Soc., 2009, 92 [9], 1901-1920.Surprisingly, embodiments of crack-free, crystalline metal oxidearticles described herein have good hydrolytic stability and pass theHydrolytic Stability Test in the Examples section, below, even in someembodiments when crack-free, crystalline metal oxide articles describedherein are subjected to the 5 hour exposure to saturated steam at 135°C. under a pressure of 0.2 MPa, one, two, three, four, or even at leastfive additional times.

Exemplary uses of crack-free, crystalline metal oxide articles describedherein include optical windows, implants (e.g. tooth implants,artificial hip, and knee joints), and dental articles, especially dentalceramic articles (e.g., restoratives, replacements, inlays, onlays,veneers, full and partial crowns, bridges, implants, implant abutments,copings, anterior fillings, posterior fillings, and cavity liner, andbridge frameworks), and orthodontic appliances (e.g., brackets, buccaltubes, cleats, and buttons). Other applications may include where acombinations of high strength, translucency, high temperature stability,low to no hydrothermal degradation, high refractive index and/or lowsintering temperatures are desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary continuous hydrothermal reactor system;

FIG. 2 is a total transmittance versus wavelength for various Examplesand Comparative Examples;

FIG. 3 is a diffuse transmittance versus wavelength for various Examplesand Comparative Examples; and

FIG. 4 is a cross-sectional view of an exemplary dental restoration.

DETAILED DESCRIPTION Sols

The zirconia sols are dispersions of zirconia based ceramic particles.The zirconia in the zirconia-based ceramic particles is crystalline, andhas been observed to be cubic, tetragonal, monoclinic, or a combinationthereof. Because the cubic and tetragonal phases are difficult todifferentiate using x-ray diffraction techniques, these two phases aretypically combined for quantitative purposes and are referred to as thecubic/tetragonal phase. “Cubic/tetragonal” or “C/T” is usedinterchangeably to refer to the cubic plus the tetragonal crystallinephases. The percent cubic/tetragonal phase can be determined, forexample, by measuring the peak area of the x-ray diffraction peaks foreach phase and using Equation (I).

% C/T=100(C/T)÷(C/T+M)  (I)

In Equation (I), C/T refers to the peak area of the diffraction peak forthe cubic/tetragonal phase, M refers to the peak area of the diffractionpeak for the monoclinic phase, and % C/T refers to the weight percentcubic/tetragonal crystalline phase. The details of the x-ray diffractionmeasurements are described more fully in the Example section below.

Typically, at least 50 (in some embodiments, at least 55, 60, 65, 70,75, 80, 85, 90, or at least 95) weight percent of the zirconia-basedparticles are present in the cubic or tetragonal crystal structure(i.e., cubic crystal structure, tetragonal crystal structure, or acombination thereof). A greater content of the cubic/tetragonal phase isoften desired.

For example, cubic/tetragonal crystals have been observed to beassociated with the formation of low aspect ratio primary particleshaving a cube-like shape when viewed under an electron microscope. Thisparticle shape tends to be relatively easily dispersed into an liquidmatrix. Typically, the zirconia particles have an average primaryparticle size is up to 50 nm (in some embodiments, up to 40 nm, 30 nm,25 nm, 20 nm, or even up to 15 nm), although larger sizes may also beuseful. The average primary particle size, which refers to thenon-associated particle size of the zirconia particles, can bedetermined by x-ray diffraction as described in the Example section.Zirconia sols described herein typically have primary particle size in arange of from 2 nm to 50 nm (in some embodiments, 5 nm to 50 nm, 2 nm to25 nm, 5 nm to 25 nm, 2 nm to 15 nm, or even 5 nm to 15 nm).

In some embodiments, the particles in the sol are non-associated. Insome embodiments, the particles are aggregated or agglomerated to a sizeup to 500 nm. The extent of association between the primary particlescan be determined from the volume-average particle size. Thevolume-average particle size can be measured using Photon CorrelationSpectroscopy as described in more detail in the Examples section below.Briefly, the volume distribution (percentage of the total volumecorresponding to a given size range) of the particles is measured. Thevolume of a particle is proportional to the third power of the diameter.The volume-average size is the size of a particle that corresponds tothe mean of the volume distribution. If the zirconia-based particles areassociated, the volume-average particle size provides a measure of thesize of the aggregates and/or agglomerates of primary particles. If theparticles of zirconia are non-associated, the volume-average particlesize provides a measure of the size of the primary particles. Thezirconia-based particles typically have a volume-average size of up to100 nm (in some embodiments, up to 90 nm, 80 nm, 75 nm, 70 nm, 60 nm, 50nm, 40 nm, 30 nm, 25 nm, 20 nm, or even up to 15 nm).

A quantitative measure of the degree of association between the primaryparticles in the zirconia sol is the dispersion index. As used hereinthe “dispersion index” is defined as the volume-average particle sizedivided by the primary particle size. The primary particle size (e.g.,the weighted average crystallite size) is determined using x-raydiffraction techniques and the volume-average particle size isdetermined using Photon Correlation Spectroscopy. As the associationbetween primary particles decreases, the dispersion index approaches avalue of 1 but can be somewhat higher or lower. The zirconia-basedparticles typically have a dispersion index in a range of from 1 to 7(in some embodiments, 1 to 5, 1 to 4, 1 to 3, 1 to 2.5, or even 1 to 2).

Photon Correlation Spectroscopy also can be used to calculate theZ-average primary particle size. The Z-average size is calculated fromthe fluctuations in the intensity of scattered light using a cumulativeanalysis and is proportional to the sixth power of the particlediameter. The volume-average size will typically be a smaller value thanthe Z-average size. The zirconia particles tend to have a Z-average sizethat is up to 100 nanometers (in some embodiments, up to 90 nm, 80 nm,70 nm, 60 nm, 50 nm, 40 nm, 35 nm, or even up to 30 nm).

Depending on how the zirconia-based particles are prepared, theparticles may contain at least some organic material in addition to theinorganic oxides. For example, if the particles are prepared using ahydrothermal approach, there may be some organic material attached tothe surface of the zirconia-based particles. Although not wanting to bebound by theory, it is believed that organic material originates fromthe carboxylate species (anion, acid, or both) included in the feedstockor formed as a byproduct of the hydrolysis and condensation reactions(i.e., organic material is often absorbed on the surface of thezirconia-based particles). For example, in some embodiments, thezirconia-based particles contain up to 15 (in some embodiments, up to12, 10, 8, or even up to 6) weight percent organic material, based onthe weight of the particles.

Although any of a variety of known methods can be used to provide thezirconia-based particles, preferably they are prepared usinghydrothermal technology. In one exemplary embodiment, the zirconia-basedsols are prepared by hydrothermal treatment of aqueous metal salt (e.g.,a zirconium salt, an yttrium salt, and an optional lanthanide elementsalt or aluminum salt) solutions, suspensions or a combination of them.

The aqueous metal salts, which are selected to be soluble in water, aretypically dissolved in the aqueous medium. The aqueous medium can bewater or a mixture of water with other water soluble or water misciblematerials. In addition, the aqueous metal salts and other water solubleor water miscible materials which may be present are typically selectedto be removable during subsequent processing steps and to benon-corrosive.

At least a majority of the dissolved salts in the feedstock are usuallycarboxylate salts rather than halide salts, oxyhalide salts, nitratesalts, or oxynitrate salts. Although not wanting to be bound by theory,it is believed that halide and nitrate anions in the feedstock tend toresult in the formation of zirconia-based particles that arepredominately of a monoclinic phase rather than the more desirabletetragonal or cubic phases. Further, carboxylates and/or acids thereoftend to be more compatible with an organic matrix material compared tohalides and nitrates. Although any carboxylate anion can be used, thecarboxylate anion often has no greater than 4 carbon atoms (e.g.,formate, acetate, propionate, butyrate, or a combination thereof). Thedissolved salts are often acetate salts. The feedstock can furtherinclude, for example, the corresponding carboxylic acid of thecarboxylate anion. For example, feedstocks prepared from acetate saltsoften contain acetic acid.

One exemplary zirconium salt is zirconium acetate salt, represented by aformula such as ZrO_(((4−n)/2)) ^(n+)(CH₃COO⁻)_(n), where n is in therange from 1 to 2. The zirconium ion may be present in a variety ofstructures depending, for example, on the pH of the feedstock. Methodsof making zirconium acetate are described, for example, in W. B.Blumenthal, “The Chemical Behavior of Zirconium,” pp. 311-338, D. VanNostrand Company, Princeton, N.J. (1958). Suitable aqueous solutions ofzirconium acetate are commercially available, for example, fromMagnesium Elektron, Inc., Flemington, N.J., that contain, for example,up to 17 weight percent zirconium, up to 18 weight percent zirconium, upto 20 weight percent zirconium, up to 22 weight percent, up to 24 weightpercent, up to 26 weight percent, and up to 28 weight percent zirconium,based on the total weight of the solution.

Similarly, exemplary yttrium salts, lanthanide element salts, andaluminum salts often have a carboxylate anion, and are commerciallyavailable. Because these salts are typically used at much lowerconcentration levels than the zirconium salt, however, salts other thancarboxylate salts (e.g., acetate salts) may also be useful (e.g.,nitrate salts).

The total amount of the various salts dissolved in the feedstock can bereadily determined based on the total percent solids selected for thefeedstock. The relative amounts of the various salts can be calculatedto provide the selected composition for the zirconia-based particles.

Typically, the pH of the feedstock is acidic. For example, the pH isusually less than 6, less than 5, or even less than 4 (in someembodiments, in a range from 3 to 4).

The liquid phase of the feedstock is typically predominantly water(i.e., the liquid phase is an aqueous based medium). Preferably, thewater is deionized to minimize the introduction of alkali metal ions,alkaline earth ions, or both into the feedstock. Optionally,water-miscible organic co-solvents are included in the liquid phase inamounts, for example, up 20 weight percent, based on the weight of theliquid phase. Suitable co-solvents include 1-methoxy-2-propanol,ethanol, isopropanol, ethylene glycol, N,N-dimethylacetamide, andN-methyl pyrrolidone.

Although, the feedstock typically is a solution and does not containdispersed or suspended solids (e.g., seed particles usually are notpresent in the feedstock), the feedstock often contains greater than 5(in some embodiments, greater than 10, 11, 12, 13, 14, 15, or up to 19,20, 21, 22, 23, 24, or 25; in some embodiments, in a range from 10 to25, 12 to 22, 14 to 20 weight percent, or even 15 to 19) weight percentsolids and these solids are typically dissolved. As used herein, the“weight percent solids” is calculated by drying a sample at 120° C., andrefers the portion of the feedstock that is not water, a water-miscibleco-solvent, or another compound that can be vaporized at temperatures upto 120° C. The weight percent solids is equal to

100(dry weight)÷(wet weight).

In this equation, the term “wet weight” refers to the weight of afeedstock sample before drying and the term “dry weight” refers to theweight of the sample after drying, for example, at 120° C. for at least30 minutes. When subjected to hydrothermal treatment, the variousdissolved salts in the feedstock undergo hydrolysis and condensationreactions to form the zirconia-based particles. These reactions areoften accompanied with the release of an acidic byproduct. That is, thebyproduct is often one or more carboxylic acids corresponding to thezirconium carboxylate salt plus any other carboxylate salt in thefeedstock. For example, if the salts are acetate salts, acetic acid isformed as a byproduct of the hydrothermal reaction.

Any suitable hydrothermal reactor can be used for the preparation of thezirconia-based particles. The reactor can be a batch or continuousreactor. The heating times are typically shorter and the temperaturesare typically higher in a continuous hydrothermal reactor compared to abatch hydrothermal reactor. The time of the hydrothermal treatments canbe varied depending, for example, on the type of reactor, thetemperature of the reactor, and the concentration of the feedstock. Thepressure in the reactor can be autogeneous (i.e., the vapor pressure ofwater at the temperature of the reactor), can be hydraulic (i.e., thepressure caused by the pumping of a fluid against a restriction), or canresult from the addition of an inert gas such as nitrogen or argon.Suitable batch hydrothermal reactors are available, for example, fromParr Instruments Co., Moline, Ill. Some suitable continuous hydrothermalreactors are described, for example, in U.S. Pat. No. 5,453,262 (Dawsonet al.) and U.S. Pat. No. 5,652,192 (Matson et al.); Adschiri et al., J.Am. Ceram. Soc., 75, 1019-1022 (1992); and Dawson, Ceramic Bulletin, 67(10), 1673-1678 (1988).

If a batch reactor is used to form the zirconia-based particles, thetemperature is often in a range from 160° C. to 275° C. (in someembodiments, 160° C. to 250° C., 170° C. to 250° C., 175° C. to 250° C.,200° C. to 250° C., 175° C. to 225° C., 180° C. to 220° C., 180° C. to215° C., or even, for example, 190° C. to 210° C.). Typically, thefeedstock is typically placed in the batch reactor at room temperature.The feedstock within the batch reactor is heated to the designatedtemperature and held at that temperature for at least 30 minutes (insome embodiments, at least 1 hour, at least 2 hours, or even at least 4hours), and up to 24 hours), (in some embodiments, up to 20 hours, up to16 hours, or up to 8 hours). For example, the temperature can be held inthe range from 0.5 to 24 hours (in some embodiments, in the range from 1to 18 hours, 1 to 12 hours, or even 1 to 8 hours). Any of a varietysized batch reactor can be used. For example, the volume of the batchreactor can be in a range from several milliliters to several liters ormore.

In some embodiments, the feedstock is passed through a continuoushydrothermal reactor. As used herein, the term “continuous” withreference to the hydrothermal reactor system means that the feedstock iscontinuously introduced and an effluent is continuously removed from theheated zone. The introduction of feedstock and the removal of theeffluent typically occur at different locations of the reactor. Thecontinuous introduction and removal can be constant or pulsed.

One exemplary continuous hydrothermal reactor system 100 is shownschematically in FIG. 1. Feedstock 110 is contained within feedstocktank 115. Feedstock tank 115 is connected with tubing or piping 117 topump 120. Similar tubing or piping can be used to connect othercomponents of the tubular reactor system. Tubing or piping 117 can beconstructed of any suitable material such as metal, glass, ceramic, orpolymer. Tubing or piping 117 can be, for example, polyethylene tubingor polypropylene tubing in the portions of continuous hydrothermalreactor system 100 that are not heated and that are not under highpressure. Any tubing that is heated or under pressure is often made ofmetal (e.g., stainless steel, carbon steel, titanium, nickel, or thelike) or has a metal outer housing. Pump 120 is used to introducefeedstock 110 into tubular reactor 130. That is, pump 120 is connectedto the inlet of tubular reactor 130. Any type of pump 120 can be usedthat is capable of pumping against the pressure within tubular reactor130. The pump can provide a constant or pulsed flow of the feedstocksolution into tubular reactor 130.

As used herein, the term “tubular reactor” refers to the portion of thecontinuous hydrothermal reactor system that is heated (i.e., the heatedzone). Although tubular reactor 130 is shown in FIG. 1 as a coil oftubing, the tubular reactor can be in any suitable shape. The shape ofthe tubular reactor is often selected based on the desired length of thetubular reactor and the method used to heat the tubular reactor. Forexample, the tubular reactor can be straight, U-shaped, or coiled. Theinterior portion of the tubular reactor can be empty or can containbaffles, balls, or other known mixing techniques.

As shown in FIG. 1, tubular reactor 130 is placed in heating medium 140within heating medium vessel 150. Heating medium 140 can be, forexample, an oil, sand, salt, or the like, that can be heated to atemperature above the hydrolysis and condensation temperatures of thezirconium. Suitable oils include plant oils (e.g., peanut oil and canolaoil). Some plant oils are preferably kept under nitrogen when heated toprevent or minimize oxidation of the oils. Other suitable oils includepolydimethylsiloxanes such as those commercially available fromDuratherm Extended Fluids, Lewiston, N.Y., under the trade designation“DURATHERM S”. Suitable salts include, for example, sodium nitrate,sodium nitrite, potassium nitrate, or mixtures thereof. Heating mediumvessel 150 can be any suitable container that can hold the heatingmedium and that can withstand the heating temperatures used for tubularreactor 130. Heating medium vessel 150 can be heated using any suitablemeans. In many embodiments, heating medium vessel 150 is positionedinside an electrically heated coil. Other types of heaters that can beused in place of heating vessel 150 and/or heating medium 140 includeinduction heaters, microwave heaters, fuel-fired heaters, heating tape,and steam coils.

Tubular reactor 130 can be made of any material capable of withstandingthe temperatures and pressures used to prepare zirconia particles.Tubular reactor 130 preferably is constructed of a material that canresist dissolution in an acidic environment. For example, carboxylicacids can be present in the feedstock or can be produced as a reactionbyproduct within the continuous hydrothermal reactor system. In someexemplary embodiments, the tubular reactor is made of stainless steel,nickel, titanium, or carbon-based steel.

In other exemplary embodiments, an interior surface of the tubularreactor contains a fluorinated polymeric material. This fluorinatedpolymeric material can include a fluorinated polyolefin. In someembodiments, the polymeric material is polytetrafluoroethylene (PTFE)such as that available under the trade designation “TEFLON” from DuPont,Wilmington, Del. Some tubular reactors have a PTFE hose within a metalhousing such as a braided stainless steel housing. These carboxylicacids can leach metals from some known hydrothermal reactors such asthose constructed of stainless steel.

The second end of tubular reactor 130 is usually connected to coolingdevice 160. Any suitable cooling device 160 can be used. In someembodiments, cooling device 160 is a heat exchanger that includes asection of tubing or piping that has an outer jacket filled with acooling medium such as cool water. In other embodiments, cooling device160 includes a coiled section of tubing or piping that is placed in avessel that contains cooling water. In either of these embodiments, thetubular reactor effluent is passed through the section of tubing and iscooled from the tubular reactor temperature to a temperature no greaterthan 100° C. (in some embodiments, no greater than 80° C., 60° C., oreven no greater than 40° C.). Other cooling devices that contain dry iceor refrigeration coils can also be used. After cooling, the reactoreffluent can be discharged into product collection vessel 180. Thereactor effluent is preferably not cooled below the freezing point priorto being discharged into product collection vessel 180.

The pressure inside the tubular reactor can be at least partiallycontrolled with backpressure valve 170, which is generally positionedbetween cooling device 160 and sample collection vessel 180.Backpressure valve 170 controls the pressure at the exit of continuoushydrothermal reactor system 100 and helps to control the pressure withintubular reactor 130. The backpressure is often at least 100 pounds persquare inch (0.7 MPa) (in some embodiments, at least 200 pounds persquare inch (1.4 MPa), 300 pounds per square inch (2.1 MPa), 400 poundsper square inch (2.8 MPa), 500 pounds per square inch (3.5 MPa), 600pounds per square inch (4.2 MPa), or even at least 700 pounds per squareinch (4.9 MPa). The backpressure should be high enough to preventboiling within the tubular reactor.

The dimensions of tubular reactor 130 can be varied and, in conjunctionwith the flow rate of the feedstock, can be selected to provide suitableresidence times for the reactants within the tubular reactor. Anysuitable length tubular reactor can be used provided that the residencetime and temperature are sufficient to convert the zirconium in thefeedstock to zirconia-based particles. The tubular reactor often has alength of at least 0.5 meter (in some embodiments, at least 1 meter, 2meters, 5 meters, 10 meters, 15 meters, 20 meters, 30 meters, 40 meters,or even at least 50 meters). The length of the tubular reactor in someembodiments is less than 500 meters (in some embodiments, less than 400meters, 300 meters, 200 meters, 100 meters, 80 meters, 60 meters, 40meters, or even less than 20 meters).

Tubular reactors with a relatively small inner diameter are typicallypreferred. For example, tubular reactors having an inner diameter nogreater than about 3 centimeters are often used because of the fast rateof heating of the feedstock that can be achieved with these reactors.Also, the temperature gradient across the tubular reactor is less forreactors with a smaller inner diameter compared to those with a largerinner diameter. The larger the inner diameter of the tubular reactor,the more this reactor resembles a batch reactor. However, if the innerdiameter of the tubular reactor is too small, there is an increasedlikelihood of the reactor becoming plugged or partially plugged duringoperation resulting from deposition of material on the walls of thereactor. The inner diameter of the tubular reactor is often at least 0.1cm (in some embodiments, at least 0.15 cm, 0.2 cm, 0.3 cm, 0.4 cm, 0.5cm, or even at least 0.6 cm). In some embodiments, the diameter of thetubular reactor is no greater than 3 cm (in some embodiments, no greaterthan 2.5 cm, 2 cm, 1.5 cm, or even greater than 1 centimeter; in someembodiments, in a range from 0.1 to 2.5 cm, 0.2 cm to 2.5 cm, 0.3 cm to2 cm, 0.3 cm to 1.5 cm, or even 0.3 cm to 1 cm).

In a continuous hydrothermal reactor, the temperature and the residencetime are typically selected in conjunction with the tubular reactordimensions to convert at least 90 mole percent of the zirconium in thefeedstock to zirconia-based particles using a single hydrothermaltreatment. That is, at least 90 mole percent of the dissolved zirconiumin the feedstock is converted to zirconia-based particles within asingle pass through the continuous hydrothermal reactor system.

Alternatively, for example, a multiple step hydrothermal process can beused. For example, the feedstock can be subjected to a firsthydrothermal treatment to form a zirconium-containing intermediate and aby-product such as a carboxylic acid. A second feedstock can be formedby removing at least a portion of the by-product of the firsthydrothermal treatment from the zirconium-containing intermediate. Thesecond feedstock can then be subjected to a second hydrothermaltreatment to form a sol containing the zirconia-based particles. Furtherdetails on this process are described, for example, in U.S. Pat. No.7,241,437 (Davidson et al.).

If a two step hydrothermal process is used, the percent conversion ofthe zirconium-containing intermediate is typically in a range from 40 to75 mole percent. The conditions used in the first hydrothermal treatmentcan be adjusted to provide conversion within this range. Any suitablemethod can be used to remove at least part of the by-product of thefirst hydrothermal treatment. For example, carboxylic acids such asacetic acid can be removed by a variety of methods such as vaporization,dialysis, ion exchange, precipitation, and filtration.

When referring to a continuous hydrothermal reactor, the term “residencetime” means the average length of time that the feedstock is within theheated portion of the continuous hydrothermal reactor system. For thereactor depicted in FIG. 1, the residence time is the average time thefeedstock is within tubular reactor 130 and is equal to the volume ofthe tubular reactor divided by the flow rate of the feedstock throughthe tubular reactor. The residence time in the tubular reactor can bevaried by altering the length or diameter of the tubular reactor as wellas by altering the flow rate of the feedstock. In some embodiments, theresidence time is at least 1 minute (in some embodiments, at least 2minutes, 4 minutes, 6 minutes, 8 minutes, or even at least 10 minutes),is typically no greater than 240 minutes (in some embodiments, nogreater than 180 minutes, 120 minutes, 90 minutes, 60 minutes, 45minutes, or even no greater than 30 minutes. In some embodiments, theresidence time is in the range from 1 to 240 minutes, 1 to 180 minutes,1 to 120 minutes, 1 to 90 minutes, 1 to 60 minutes, 10 to 90 minutes, 10to 60 minutes, 20 to 60 minutes, or even 30 to 60 minutes.

Any suitable flow rate of the feedstock through the tubular reactor canbe used as long as the residence time is sufficiently long to convertthe dissolved zirconium to zirconia-based particles. That is, the flowrate is often selected based on the residence time needed to convert thezirconium in the feedstock to zirconia-based particles. Higher flowrates are desirable for increasing throughput and for minimizing thedeposition of materials on the walls of the tubular reactor. A higherflow rate can often be used when the length of the reactor is increasedor when both the length and diameter of the reactor are increased. Theflow through the tubular reactor can be either laminar or turbulent.

In some exemplary continuous hydrothermal reactors, the reactortemperature is in the range from 170° C. to 275° C., 170° C. to 250° C.,170° C. to 225° C., 180° C. to 225° C., 190° C. to 225° C., 200° C. to225° C., or even 200° C. to 220° C. If the temperature is greater thanabout 275° C., the pressure may be unacceptably high for somehydrothermal reactors systems. However, if the temperature is less thanabout 170° C., the conversion of the zirconium in the feedstock tozirconia-based particles may be less than 90 weight percent usingtypical residence times.

The effluent of the hydrothermal treatment (i.e., the product of thehydrothermal treatment) is a zirconia-based sol. The sol contains atleast 3 weight percent zirconia-based particles dispersed, suspended, ora combination thereof in an aqueous medium. In some embodiments, thezirconia-based particles can contain (a) 0 to 5 mole percent of alanthanide element oxide, based on total moles of inorganic oxide in thezirconia-based particles, and (b) 1 to 15 mole percent yttrium oxide,based on total moles of inorganic oxide in the zirconia-based particles.The zirconia-based particles are crystalline and have an average primaryparticle size no greater than 50 nanometers. In some embodiments, ceriumoxide, magnesium oxide, ytterbium oxide, and/or calcium oxide may beused with or in place of the yttria.

The sol effluent of the hydrothermal treatment usually containsnon-associated zirconia-based particles. The effluent is typically clearor slightly cloudy. By contrast, zirconia-based sols that containagglomerated or aggregated particles usually tend to have a milky orcloudy appearance. The zirconia-based sols often have a high opticaltransmission due to the small size and non-associated form of theprimary zirconia particles in the sol. High optical transmission of thesol can be desirable in the preparation of transparent or translucentcomposite materials. As used herein, “optical transmission” refers tothe amount of light that passes through a sample (e.g., a zirconia-basedsol) divided by the total amount of light incident upon the sample. Thepercent optical transmission may be calculated using the equation

100(I/I _(O))

where I is the light intensity passing though the sample and I_(O) isthe light intensity incident on the sample. The optical transmission maybe determined using an ultraviolet/visible spectrophotometer set at awavelength of 600 nanometers with a 1 centimeter path length. Theoptical transmission is a function of the amount of zirconia in a sol.For zirconia-based sols having about 1 weight percent zirconia, theoptical transmission is typically at least 70 percent (in someembodiments, at least 80 percent, even or at least 90 percent). Forzirconia-based sols having about 10 weight percent zirconia, the opticaltransmission is typically at least 20 percent (in some embodiments, atleast 50 percent, or even at least 70 percent).

In some embodiments, at least a portion of the aqueous-based medium isremoved from the zirconia-based sol. Any known means for removing theaqueous-based medium can be used. This aqueous-based medium containswater and often contains dissolved carboxylic acids and/or anionsthereof that are present in the feedstock or that are byproducts of thereactions that occur within the hydrothermal reactor. As used herein,the term “carboxylic acids and/or anions thereof” refers to carboxylicacids, carboxylate anions of these carboxylic acids, or mixturesthereof. The removal of at least a portion of these dissolved carboxylicacids and/or anions thereof from the zirconia-based sol may be desirablein some embodiments. The zirconia-based sol can be subjected, forexample, to at least one of vaporization, drying, ion exchange, solventexchange, diafiltration, or dialysis, for example, for concentrating,removal of impurities or to compatibilize with other components presentin the sol.

In some embodiments, the zirconia sol (prepared from hydrothermalprocess or other processes) is concentrated. Along with removing atleast a portion of the water present in the effluent, the concentrationor drying process often results in the vaporization of at least aportion of the dissolved carboxylic acids.

In other embodiments, for example, the zirconia based sol can besubjected to dialysis or diafiltration. Dialysis and diafiltration bothtend to remove at least a portion of the dissolved carboxylic acidsand/or anions thereof. For dialysis, a sample of the effluent can bepositioned within a membrane bag that is closed and then placed within awater bath. The carboxylic acid and/or carboxylate anions diffuse out ofthe sample within the membrane bag. That is, these species will diffuseout of the effluent through the membrane bag into the water bath toequalize the concentration within the membrane bag to the concentrationin the water bath. The water in the bath is typically replaced severaltimes to lower the concentration of species within the bag. A membranebag is typically selected that allows diffusion of the carboxylic acidsand/or anions thereof but does not allow diffusion of the zirconia-basedparticles out of the membrane bag.

For diafiltration, a permeable membrane is used to filter the sample.The zirconia particles can be retained by the filter if the pore size ofthe filter is appropriately chosen. The dissolved carboxylic acidsand/or anions thereof pass through the filter. Any liquid that passesthrough the filter is replaced with fresh water. In a discontinuousdiafiltration process, the sample is often diluted to a pre-determinedvolume and then concentrated back to the original volume byultrafiltration. The dilution and concentration steps are repeated oneor more times until the carboxylic acid and/or anions thereof areremoved or lowered to an acceptable concentration level. In a continuousdiafiltration process, which is often referred to as a constant volumediafiltration process, fresh water is added at the same rate that liquidis removed through filtration. The dissolved carboxylic acid and/oranions thereof are in the liquid that is removed.

While the majority of the yttrium and lanthanum, if present, areincorporated into the crystalline zirconia particles there is a fractionof these metals that can be removed during the diafiltration or dialysisprocess. The actual composition of a sol after diafiltration may bedifferent than that before dialysis. For example, a sol produced with a97.5:2.3:2 ZrO₂:Y₂O₃:La₂O₃ composition was observed to result in a solwith the composition 96.6:2.2:1.3 ZrO₂:Y₂O₃:La₂O₃ after the dialysis. Inanother example, a sol prepared with a 88:12 ZrO₂/Y₂O₃ composition wasobserved to result in a sol with the composition 90.7:9.3 ZrO₂/Y₂O₃after the dialysis. The actual composition of the final sol andcomposites made from these can be calculated from these data and rule ofmixtures.

A zirconia based sol comprises zirconia-based particles dispersed and/orsuspended (i.e., dispersed, suspended, or a combination thereof) in anaqueous/organic matrix. In some embodiments, the zirconia-basedparticles can be dispersed and/or suspended in the organic matrixwithout any further surface modification. The organic matrix can beadded directly to zirconia based sol. Also, for example, the organicmatrix can be added to the zirconia based sol after treatment to removeat least some of the water, after treatment to remove at least some ofthe carboxylic acids and/or anions thereof, or after both treatments.The organic matrix that is added is often contains a polymerizablecomposition that is subsequently polymerized and/or crosslinked to forma gel.

In some embodiments, the zirconia based sol can be subjected to asolvent exchange process. An organic solvent having a higher boilingpoint than water can be added to the effluent. Examples of organicsolvents that are suitable for use in a solvent exchange method include1-methoxy-2-propanol and N-methyl pyrrolidone. The water then can beremoved by a method such as distillation, rotary evaporation, or ovendrying. Depending on the conditions used for removing the water, atleast a portion of the dissolved carboxylic acid and/or anion thereofcan also be removed. Other organic matrix material can be added to thetreated effluent (i.e., other organic matrix material can be added tothe zirconia-based particle suspended in the organic solvent used in thesolvent exchange process).

In some embodiments, the zirconia-based sols are treated with a surfacemodification agent to improve compatibility with the organic matrixmaterial. Surface modification agents may be represented by the formulaA-B, where the A group is capable of attaching to the surface of azirconia-based particle and B is a compatibility group. Group A can beattached to the surface by adsorption, formation of an ionic bond,formation of a covalent bond, or a combination thereof. Group B can bereactive or nonreactive and often tends to impart characteristics to thezirconia-based particles that are compatible (i.e., miscible) with anorganic solvent, with another organic matrix material (e.g., monomer,oligomers, or polymeric material), or both. For example, if the solventis non-polar, group B is typically selected to be non-polar as well.Suitable B groups include linear or branched hydrocarbons that arearomatic, aliphatic, or both aromatic and aliphatic. The surfacemodifying agents include carboxylic acids and/or anions thereof,sulfonic acids and/or anions thereof, phosphoric acids and/or anionsthereof, phosphonic acids and/or anions thereof, silanes, amines, andalcohols. Suitable surface modification agents are further described,for example, in PCT Application Publication WO 2009/085926 (Kolb etal.), the disclosure of which is incorporated herein by reference.

A surface modification agent can be added to the zirconia-basedparticles using conventional techniques. The surface modification agentcan be added before or after any removal of at least a portion of thecarboxylic acids and/or anions thereof from the zirconia-based sol. Thesurface modification agent can be added before or after removal of thewater from the zirconia-based sol. The organic matrix can be addedbefore or after surface modification or simultaneously with surfacemodification. Various methods of adding the surface modification agentare further described, for example, in WO 2009/085926 (Kolb et al.), thedisclosure of which is incorporated herein by reference.

The surface modification reactions can occur at room temperature (e.g.,20° C. to 25° C.) or at an elevated temperature (e.g., up to about 95°C.). When the surface modification agents are acids such as carboxylicacids, the zirconia-based particles typically can be surface-modified atroom temperature. When the surface modification agents are silanes, thezirconia-based particles are typically surface modified at elevatedtemperatures.

The organic matrix typically includes a polymeric material or aprecursor to a polymeric material such as a monomer or an oligomerhaving a polymerizable group and a solvent. The zirconia-based particlescan be combined with the organic matrix using conventional techniques.For example, if the organic matrix is a precursor to a polymericmaterial, the zirconia-based particles can be added prior to thepolymerization reaction. The composite material containing a precursorof a polymeric material is often shaped before polymerization.

Representative examples of monomers include (meth)acrylate-basedmonomers, styrene-based monomers, and epoxy-based monomers.Representative examples of reactive oligomers include, polyesters having(meth)acrylate groups, polyurethanes having (meth)acrylate groups,polyethers having (meth)acrylate groups, or acrylics. Representativeexamples of polymeric material include polyurethanes,poly(meth)acrylates, and polystyrenes.

Gels

The zirconia based sols are typically solidified by gelation.Preferably, the gelation process allows large gels to be formed withoutcracks and gels that can be further processed without inducing cracks.For example, preferably, the gelation process leads to a gel having astructure that will not collapse when the solvent is removed. The gelstructure is compatible with and stable in a variety of solvents andconditions that may be necessary for supercritical extraction.Furthermore, the gel structure needs to be compatible with supercriticalextraction fluids (e.g., supercritical CO₂). In other words, the gelsshould be stable and strong enough to withstand drying, so as to producestable gels and give materials that can be heated to burn out theorganics, pre-sintered, and densified without inducing cracks.Preferably, the resulting gels have relatively small and uniform poresize to aid in sintering them to high density at low sinteringtemperatures. However, preferably the pores of the gels are large enoughto allow product gases of organic burnout escape without leading tocracking of the gel. Furthermore, the gelation step allows control ofthe density of the resulting gels aids in the subsequent processing ofthe gel such as supercritical extraction, organic burnout, andsintering. It is preferable that the gel contain the minimum amount oforganic material or polymer modifiers.

The gels described herein contain zirconia-based particles. In someembodiments, the gels contain at least two types of zirconia-basedparticles varying in crystalline phases, composition, or particle size.We have found, particulate based gels can lead to less shrinkagecompared to gels produced form alkoxides which undergo significant andcomplicated condensation and crystallization reactions during furtherprocessing. The crystalline nature allows combinations of differentcrystal phases on a nanoscale. Applicants have observed that formationof a gel thru polymerization of these reactive particles yield strong,resilient gels. Applicants have also found that the use of mixtures ofsols with crystalline particles can allow formation of stronger and moreresilient gels for further processing. For example, Applicants observedthat a gel comprising a mixture of cubic and tetragonal zirconiaparticles was less susceptible to cracking during supercriticalextraction and organic burnout steps.

The gels comprise organic material and crystalline metal oxideparticles, wherein the crystalline metal oxide particles are present ina range from 3 to 20 volume percent, based on the total volume of thegel, wherein at least 70 (in some embodiments, at least 75, 80, 85, 90,95, 96, 97, 98, or even at least 99; in a range from 70 to 99, 75 to 99,80 to 99, or even 85 to 99) mole percent of the crystalline metal oxideis ZrO₂. Optionally, the gels may also include amorphous non-crystallineoxide sources.

In some embodiments, gels described herein, the crystalline metal oxideparticles have an average primary particle size in a range from 5nanometers to 50 nanometers (in some embodiments, in a range from 5nanometers to 25 nanometers, 5 nanometers to 15 nanometers, or even from5 nanometers to 10 nanometers). Typically, the average primary particlesize is measured by using the X-Ray Diffraction technique. Preferably,the particles are not agglomerated but, it is possible that particleswith some degree of aggregation may also be useful.

Exemplary sources of the ZrO₂, Y₂O₃, La₂O₃, and Al₂O₃ includecrystalline zirconia based sols prepared by any suitable means. The solsdescribed above are particularly well suited. The Y₂O₃, La₂O₃, andAl₂O₃, can be present in the zirconia based particles, and/or present asseparate colloidal particles or soluble salts.

In some embodiments, for gels described herein the crystalline metaloxide particles comprise a first plurality of particles, and a second,different plurality of particles (i.e., is distinguishable by averagecomposition, phase(s), microstructure, and/or size).

Typically, gels described herein have an organic content that is atleast 3 (in some embodiments, at least 4, 5, 10, 15, or even at least20) percent by weight, based on the total weight of the gel. In someembodiments, gels described herein have an organic content in a rangefrom 3 to 30, 10 to 30, or even 10 to 20, percent by weight, based onthe total weight of the gel.

Optionally, gels described herein comprise at least one of Y₂O₃ (e.g.,in a range from 1 to 15, 1 to 9, 1 to 5, 6 to 9, 3.5 to 4.5, or even 7to 8 mole percent of the crystalline metal oxide is Y₂O₁), La₂O₃ (e.g.,up to 5 mole percent La₂O₃), or Al₂O₃ (e.g., up to 0.5 mole percentAl₂O₃). In one exemplary gel the crystalline metal oxide comprises in arange from 1 to 5 mole percent Y₂O₃, and in a range from 0 to 2 molepercent La₂O₃, and in a range from 93 to 97 mole percent ZrO₂. Inanother exemplary gel the crystalline metal oxide comprises in a rangefrom 6 to 9 mole percent Y₂O₃, and in a range from 0 to 2 mole percentLa₂O₃, and in a range from 89 to 94 mole percent ZrO₂. In anotherexemplary gel the crystalline metal oxide comprises in a range from 3.5to 4.5 mole percent Y₂O₃, and in a range from 0 to 2 mole percent La₂O₃,and in a range from 93.5 to 96.5 mole percent ZrO₂. In another exemplarygel the crystalline metal oxide comprises in a range from 7 to 8 molepercent Y₂O₃, and in a range from 0 to 2 mole percent La₂O₃, and in arange from 90 to 93 mole percent ZrO₂. Other optional oxides that may bepresent in gels described herein include at least one of CeO₂, Pr₂O_(3.)Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃,Yb₂O₃, Fe₂O₃, MnO₂, CO₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, or Lu₂O₃.Additives that may add desired coloring to the resulting crack freecrystalline metal oxide articles include at least one of Fe₂O₃, MnO₂,Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, Er₂O₃, Pr₂O₃, Eu₂O₃, Dy₂O₃, Sm₂O₃,V₂O₅, W₂O₅ or CeO₂. In some embodiments, the amount of optional oxide(s)is in an amount in a range from about 10 ppm to 20,000 ppm. In someembodiments, it is desirable to have sufficient oxides present to so thecrack free crystalline metal oxide articles has coloring of a tooth.

One exemplary method for making gels described herein comprisesproviding a first zirconia sol comprising crystalline metal oxideparticles having an average primary particle size of not greater than 15nanometers (in some embodiments, in a range from 5 nanometers to 15nanometers), wherein at least 70 (in some embodiments, at least 75, 80,85, 90, 95, 96, 97, 98, or even at least 99; in a range from 70 to 99,75 to 99, 80 to 99, or even 85 to 99) mole percent of the crystallinemetal oxide is ZrO₂. The sol is optionally concentrated to provide aconcentrated zirconia sol. A co-solvent, surface modifiers and optionalmonomers are added while stirring to obtain a well dispersed sol. Also,a radical initiator (e.g., ultraviolet (UV) or thermal initiator) isadded to the radically polymerizable surface-modified zirconia sol. Theresulting sol is optionally purged with N₂ gas to remove oxygen. Theresulting sol can be gelled by radiating with actinic or heating at atleast one temperature for a time sufficient to polymerize the radicallysurface-modified zirconia sol comprising the radical initiator to form agel. Typically the resulting gel is a strong, translucent gel.

In some embodiments the sols for making aerogels described hereincomprise zirconia based particles that are surface modified with aradically polymerizable surface treatment agent/modifier. The sol can begelled, for example, by radical (thermal initiation or light initiation)polymerization. Exemplary radically polymerizable surface modifiersinclude acrylic acid, methacrylic acid, beta-carboxyethyl acrylate, andmono-2-(methacryloxyethyl)succinate. An exemplary modification agent forimparting both polar character and reactivity to the zirconia-containingnanoparticles is mono(methacryloxypolyethyleneglycol) succinate.Exemplary polymerizable surface modifiers can be can reaction productsof hydroxyl containing polymerizable monomers with cyclic anhydridessuch as succinic anhydride, maleic anhydride and pthalic anhydride.Exemplary polymerization hydroxyl containing monomers includehyroxyethyl acrylate, hydroxyethyl methacrylate, hydoxypropyl acrylate,hydoxyproyl methacrylate, hydroxyl butyl acrylate, and hydroxybutylmethacrylate. Acyloxy and methacryloxy functional polyethylene oxide,and polypropylene oxide may also be used as the polymerizable hydroxylcontaining monomers. Exemplary polymerizable silanes includealkyltrialkoxysilanes methacryloxyalkyltrialkoxysilanes oracryloxyalkyltrialkoxysilanes (e.g.,3-methacryloxypropyltrimethoxysilane, 3-acryloxypropyltrimethoxysilane,and 3-(methacryloxy)propyltriethoxysilane; as3-(methacryloxy)propylmethyldimethoxysilane, and3-(acryloxypropyl)methyldimethoxysilane);methacryloxyalkyldialkylalkoxysilanes oracyrloxyalkyldialkylalkoxysilanes (e.g.,3-(methacryloxy)propyldimethylethoxysilane);mercaptoalkyltrialkoxylsilanes (e.g., 3-mercaptopropyltrimethoxysilane);aryltrialkoxysilanes (e.g., styrylethyltrimethoxysilane); vinylsilanes(e.g., vinylmethyldiacetoxysilane, vinyldimethylethoxysilane,vinylmethyldiethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane,vinyltriacetoxysilane, vinyltriisopropoxysilane, vinyltrimethoxysilane,and vinyltris(2-methoxyethoxy)silane).

In some embodiments, sols for making aerogels described herein comprisezirconia-based particles that are surface modified with nonreactivesurface modifiers which can impart additional compatibility towardorganic matrix. Exemplary nonreactive surface modifiers include2-[2-(2-methoxyethoxyl)ethoxy]acetic acid (MEEAA) and2-(2-methoxyethoxyl)acetic acid (MEAA). Other exemplary nonreactivesurface modifiers include the reaction product of an aliphatic oraromatic anhydride and a polyalkylene oxide mono-ether (e.g., succinicacid mono-[2-(2-methoxy-ethoxy)-ethyl] ester, maleic acidmono-[2-(2-methoxy-ethoxy)-ethyl] ester, and glutaric acidmono-[2-(2-methoxy-ethoxy)-ethyl] ester). In some embodiments, thesurface modification agent is a carboxylic acid and/or anion thereof andthe compatibility group imparts a non-polar character to thezirconia-containing nanoparticles. For example, the surface modificationagent can be a carboxylic acid and/or anion thereof having a linear orbranched aromatic group or aliphatic hydrocarbon group. Exemplarynon-polar surface modifiers include octanoic acid, dodecanoic acid,stearic acid, oleic acid, and combinations thereof. Exemplary silanesurface modifiers include such asN-(3-triethoxysilylpropyl)methoxyethoxyethoxyethyl carbamate,N-(3-triethoxysilylpropyl) methoxyethoxyethoxyethyl carbamate (availableunder the trade designation “SILQUEST A-1230” from Momentive SpecialtyChemicals. Columbus, Ohio), n-octyltrimethoxysilane,n-octyltriethoxysilane, isooctyltrimethoxysilane,dodecyltrimethoxysilane, octadecyltrimethoxysilane, andpropyltrimethoxysilane and combinations thereof.

Methods for adding a surface modification agent to thezirconia-containing nanoparticles are known in the art. The surfacemodification agent can be added, for example, before or after anyremoval of at least a portion of the carboxylic acids and/or anionsthereof from the zirconia-containing sol. The surface modification agentcan be added, for example, before or after removal of the water from thezirconia-containing sol. The organic matrix can be added, for example,after surface modification or simultaneously with surface modification.

Optionally, a radically reactive co-monomer can be incorporated into thesol to be copolymerized into the gel. The monomers can bemono-functional, difunctional or multifunctional. The monomers can havemethacrylate, acrylate or styrenic functionality. The type of monomerused may depend on solvent system used. The monomers can be stiff orflexible. Exemplary monomers include hydroxyethyl methacrylate,acrylamide, 1-vinyl-2-pyrrolidione, hydroxyethyl acrylate, and butylacrylate. Other exemplary monomers include di and multifunctionalacrylates and methacrylates (e.g., pentaerythritol tetraacrylate andpentaerythritol triacrylate (available, for example, under the tradedesignations “SARTOMER SR444” and “SARTOMER SR295” from SartomerCorporation), ethoxylated pentaerythritol tetraacrylate (available, forexample, under the trade designation “SARTOMER SR494” from SartomerCorporation), polyethylene glycol (400) dimethacrylate (available, forexample, under the trade designation “SARTOMER SR603” from SartomerCorporation), ethoxylated (3) trimethylolpropane triacrylate (available,for example, under the trade designation “SARTOMER SR454” from SartomerCorporation), ethoxylated (9) trimethylolpropane triacrylate (available,for example, under the trade designation “SARTOMER 502” from SartomerCorporation) ethoxylated (15) trimethylolpropane triacrylate (available,for example, under the trade designation “SARTOMER 9035” from SartomerCorporation), and mixtures thereof.

In one exemplary embodiment, the gel is formed by radical polymerizationof the surface modified particles and optional monomers. Thepolymerization can be initiated by any suitable means such as thermallyor actinic radiation or UV initiators. Exemplary thermal initiatorsinclude (2,2′-azobis(2-methylbutyronitrile) (available, for example,under the trade designation “VAZO 67” from E. I. du Pont de Nemours andCompany, Wilmington, Del.), azobisisobututyronitrile (available, forexample, under the trade designation “VAZO 64” from E. I. du Pont deNemours and Company), 2,2′-azodi-(2,4-Dimethylvaleronitrile (available,for example, under the trade designation “VAZO 52” from E. I. du Pont deNemours and Company), and 1,1′-azobis(cyclohexanecabonitrile)(available, for example, under the trade designation “VAZO 88” from E.I. du Pont de Nemours and Company). Peroxides and hydroperoxides (e.g.,benzoyl peroxide and lauryl peroxide) may also be useful. The initiatorselection may be influenced, for example, by solvent choice, solubilityand desired polymerization temperature. A preferred initiator is the2,2′-azobis(2-methylbutyronitrile) available from E. I. du Pont deNemours and Company under the trade designation “VAZO 67”).

Exemplary UV initiators include 1-hydroxycyclohexyl benzophenone(available, for example, under the trade designation “IRGACURE 184” fromCiba Specialty Chemicals Corp., Tarrytown, N.Y.),4-(2-hydroxyethoxyl)phenyl-(2-hydroxy-2-propyl) ketone (available, forexample, under the trade designation “IRGACURE 2529” from Ciba SpecialtyChemicals Corp.), 2-hydroxy-2-methylpropiophenone (available, forexample, under the trade designation “DAROCURE D111” from Ciba SpecialtyChemicals Corp. and bis(2,4,6-trimethylbenzoyl)-phenylposphineoxide(available, for example, under the trade designation “IRGACURE 819” fromCiba Specialty Chemicals Corp.).

In some embodiments, dissolved oxygen is removed from the zirconia-basedsols before forming the zirconia based gels. This can be accomplished,for example, by techniques known in the art, such as vacuum degassing ornitrogen gas purging. For example, the zirconia-based sol may be gelledby purging with nitrogen gas before heating. In some embodiments, thezirconia-based sol to be gelled can be placed in a mold and sealed fromthe atmosphere before further processing.

Liquid in the gel can be exchanged with a second liquid, for example, bysoaking the gel in the second liquid for a time sufficient to allow anexchange to occur. For example, water present in a gel can be removed bysoaking the gel in a dry solvent (e.g., 200 proof ethanol). In oneembodiment, gels are soaked in ethanol in an amount 10 times that of thewater present in the gel for 24 hours. The alcohol is then replaced withfresh dry solvent, and the process repeated four times. The times thegels are exposed to air should be minimized as ambient drying of thegels tends to cause cracking.

Typically the gels have x, y, z dimensions of at least 1 mm (in someembodiments, at least 3 mm, at least 5 mm, 10 mm, 15 mm, 20 mm, or evenat least 25 mm), although the specific size may depend on the intendeduse of the resulting ceramic. For example, for some dental applications,the gels have x, y, z dimensions of greater than 1 mm, 5, mm, or evengreater than 10 mm. The maximum size of the gels is limited by thepracticality of subsequent processing steps such as organic burnout andextraction.

Aerogels

Aerogels described herein are formed by removing solvent from zirconiagels described herein without excessive shrinkage (e.g., not greaterthan 10%). Any suitable gel can be used. The gels described here aparticularly well suited. The gel structure must be strong enough towithstand at least some shrinkage and cracking during the drying(solvent removal). The structure of the aerogel is essentiallyhomogeneous.

The aerogels can be prepared by drying gels via super criticalextraction. In some embodiments, the aerogels are prepared by dryinggels under supercritical conditions of the solvent used in preparing thegel.

In some embodiments, of aerogels described herein, the crystalline metaloxide particles have an average primary particle size in a range from 2nm to 50 nm (in some embodiments, 5 nm to 50 nm, 2 nm to 25 nm, 5 nm to25 nm, 2 nm to 15 nm, or even 5 nm to 15 nm). Typically, the averageprimary particle size is measured by using the X-Ray Diffractiontechnique.

In some embodiments, aerogels described herein the crystalline metaloxide particles comprise a first plurality of particles, and a second,different plurality of particles (i.e., is distinguishable by averagecomposition, phase(s), microstructure, and/or size).

Typically, aerogels described herein have an organic content that is atleast 3 (in some embodiments, at least 4, 5, 10, 15, or even at least20) percent by weight, based on the total weight of the aerogel. In someembodiments, aerogels described herein have an organic content in arange from 3 to 30, 10 to 30, or even 10 to 20 percent by weight, basedon the total weight of the aerogel.

Exemplary organic materials for making aerogel described herein includeacetic acid, acrylic acid, 2-hydroxyethyl methaacrylate, acrylamide1-vinyl-2-pyrrolidione, trimethylolpropane triacrylate and ethoxylated.Other exemplary organics are ethoxylated pentaerythritol tetraacrylate(available, for example, under the trade designations “SR351,” “SR350,”and “SR454” from Sartomer Corporation), pentaerythritol triacrylate,teteraacrylate and ethoxylated versions such as those available underthe trade designations “SR295,” “SR444,” “SR494” from SartomerCorporation. Others (meth)acrylate monomers could also possibly be usedin those formulations. Surface treatment agents include carboxlic acids,sulfonic acids, phosphonic acids, silanes. Changing to other solventsystems could expand the possible monomer choices even more. Theaerogels could also possibly contain residual solvent(s) (e.g., water,ethanol, methanol methoxypropanol).

Optionally, aerogels described herein comprise at least one of Y₂O₃(e.g., in a range from 1 to 15, 1 to 9, 1 to 5, 6 to 9, 3.5 to 4.5, oreven 7 to 8) mole percent of the crystalline metal oxide is Y₂O₃), La₂O₃(e.g., up to 5 mole percent La₂O₃), Al₂O₃ (e.g., up to 0.5 mole percentAl₂O₃). One exemplary aerogel comprises in a range from 1 to 5 molepercent of the crystalline metal oxide is Y₂O₃, and in a range from 0 to2 mole percent of the crystalline metal oxide is La₂O₃, and in a rangefrom 93 to 99 mole percent of the crystalline metal oxide is ZrO₂.Another exemplary aerogel comprises in a range from 6 to 9 mole percentof the crystalline metal oxide is Y₂O₃, and in a range from 0 to 2 molepercent of the crystalline metal oxide is La₂O₃, and in a range from 89to 94 mole percent of the crystalline metal oxide is ZrO₂. In anotherexemplary aerogel the crystalline metal oxide comprises in a range from3.5 to 4.5 mole percent Y₂O₃, and in a range from 0 to 2 mole percent ofthe crystalline metal oxide is La₂O₃, and in a range from 93.5 to 96.5mole percent ZrO₂. In another exemplary aerogel the crystalline metaloxide comprises in a range from 7 to 8 mole percent Y₂O₃, and in a rangefrom 0 to 2 mole percent of the crystalline metal oxide is La₂O₃, and ina range from 90 to 93 mole percent ZrO₂. Other optional oxides that maybe present in aerogels described herein include at least one of CeO₂,Pr₂O_(3.) Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃,Tm₂O₃, Yb₂O₃, Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, orLu₂O₃. Additives that may add desired coloring to the resulting crackfree crystalline metal oxide articles include at least one of Fe₂O₃,MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, Er₂O₃, Pr₂O₃, Eu₂O₃, Dy₂O₃,Sm₂O₃, or CeO₂. In some embodiments, the amount of optional oxide(s) isin an amount in a range from about 10 ppm to 20,000 ppm. In someembodiments, it is desirable to have sufficient oxides present to so thecrack free crystalline metal oxide articles has coloring of a tooth.

Aerogels described herein typically have a volume percent of oxide in arange of 3 to 20 (in some embodiments, 3 to 15, 3 to 14, or even 8 to14) percent. Aerogels with lower volume percents of oxide tend to bevery fragile and crack during supercritical drying or subsequentprocessing. Aerogels with higher oxide contents tend to crack duringorganic burnout because it is more difficult for volatile by-products toescape from the denser structure.

In some embodiments, aerogels described herein have a surface area (e.g.a BET surface area) in the range of 100 m²/g to 300 m²/g (in someembodiments, 150 m²/g to 250 m²/g), and a continuous pore channel size(also referred to as “average connected pore size”) in a range of 10 nmto 20 nm. In some embodiments, the structure of aerogels describedherein is a composite of oxide particles, 3 nm to 10 nm (in someembodiments, 4 nm to 8 nm) in size and organics composed of acetategroups and polymerized monomers. The amount of organic is typically 10to 20 weight percent of the aerogel.

Aerogels described herein can be made, for example, by providing a firstzirconia sol comprising crystalline metal oxide particles having anaverage primary particle size of up to 50 nm (in some embodiments, 2 nmto 50 nm, 5 nm to 25 nm, 2 nm to 15 nm, or even 5 nm to 15 nm), whereinat least 70 (in some embodiments, at least 75, 80, 85, 90, 95, 96, 97,98, or even at least 99; in a range from 70 to 99, 75 to 99, 80 to 99,or even 85 to 99) mole percent of the crystalline metal oxide is ZrO₂.The first zirconia sol is then optionally concentrated to provide aconcentrated zirconia sol. A co-solvent, surface modifiers and optionalmonomers are added while stirring to obtain a well dispersed sol,wherein the cosolvent is optional). A radical initiator (e.g.,ultraviolent (UV) or thermal initiator) is added to the radicallypolymerizable surface-modified zirconia sol. Optionally the resultingsol is purged with N₂ gas to remove oxygen. The resulting sol is thengelled by radiating with actinic or heating at at least one temperaturefor a time sufficient to polymerize the radically surface-modifiedzirconia sol comprising the radical initiator to form a gel. Typicallythe resulting gel is a strong, translucent gel. The water, if present,is then removed from the gel via alcohol exchange to provide an at leastpartially de-watered gel. The gel is then converted to an aerogel byremoving the alcohol, if present, from the partially de-watered gel viasuper critical extraction to provide the aerogel.

The liquid solvent from the at least partially de-watered gel preferablyremoved in the absence of capillary forces in the gel structure toprovide the monolithic aerogel, as even the presence of small capillaryforces during the solvent removal can result in the collapse of the gelskeleton and crack the gel. In one exemplary embodiment removing theliquid solvent comprises placing the wet, at least partially de-wateredgel in an autoclave, heating the autoclave above the criticaltemperature of the liquid solvent, pressurizing the autoclave above thecritical pressure of the liquid solvent, then slowly removing the liquidsolvent by releasing the pressure in the autoclave to about 1 bar atthat temperature (i.e., the applicable critical temperature) to providethe monolithic aerogel.

In one exemplary embodiment, removing the first liquid solvent from theat least partially de-watered gel comprises replacing the first liquidsolvent with a second liquid solvent, then slowly increasing thetemperature and pressure of the at, least partially de-watered gelsuntil supercritical conditions for the second solvent are obtained, thenslowly releasing the pressure to about 1 bar to provide the monolithicaerogel.

In some embodiments, the complete exchange of the first liquid solventwith the second solvent is carried out under supercritical conditions.In some embodiments, the first liquid solvent is miscible with thesecond solvent. This method comprises placing the at least partiallyde-watered gel into a pressure vessel with a sufficient volume of thefirst liquid solvent to completely immerse the gel, pumping the secondsolvent into the autoclave at a temperature above the criticaltemperature of the second solvent until a pressure greater than thecritical pressure of the second solvent is reached, maintaining thesupercritical pressure in the pressure vessel for a time sufficient tocomplete the solvent exchange by pumping an additional quantity of thesecond solvent into the pressure vessel while simultaneously venting themixture of the first and second solvents to a separator vessel, thenslowly releasing the pressure to 1 bar to provide the monolithicaerogel. Typically, the second solvent is carbon dioxide. Exemplaryfirst liquid solvents include methanol, ethanol, isopropanol,β-methoxyethanol, β-ethoxyethanol, methoxypropanol, t-butyl alcohol,sec-butyl alcohol, t-amyl alcohol, hexanol, cyclohexanol, cyclohexane,heptane, dodecane, formic acid, acetic acid, hexanoic cid, isohexanoicacid, octanoic acid, acetal, acetaldehyde, acetic anhydride, acetone,acetonitrile, acetophenone, acetyl chloride, acrolein, acetonitrile,benzene, benzaldehyde, benzonitrile, benzoyl chloride, 2-butanone,n-butyl ether, camphor, carbon disulfide, carbon tetrachloride,chloroacetone, chlorobenzene, chloroform, cyclohexanone, 1-decene,p-dichlorobenzene, diethylene glycol monoethyl ether,N,N-diethylacetamide, N,N-dimethylacetamide, N,N-dimethylformamide,N,N-diethylformamide, 2,2-dimethylpentane, p-dioxane, ethyl acetate,ethyl acetoacetate, ethyl benzoate, ethyl carbonate, ethylchloroacetate, ethyl chloroformate, ethylene bromide, ethylenediformate, ethylene glycol monobutyl ether, ethyl ether, ethyl formate,ethyl lactate, ethyl maleate, ethyl oxalate, ethyl phenylacetate, ethylsalicylate, ethyl succinate, ethyl sulfate, furfural, 1-heptaldehyde,2,5-hexanedione, indene, isopropyl ether, limonene, methyl acetate,methylal, methyl benzoate, methylcyclohexane, methyl formate, methylsalicylate, methyl sulfate, nitrobenzene, nitroethane, nitromethane,o-nitrophenol, p-nitrotoluene, 1-nitropropane, 2-octanone, thioxane,paraldehyde, pentanaldehyde, 2-picoline, pinene, propionaldehyde,pyridine, salicylaldehyde, thiophene, toluene, triacetin,tri-sec-butylbenzene, and 2,2,3-trimethylbutane. Preferred first liquidsolvents include methanol, ethanol, and methoxypropanol.

Further information on the principles and practice of super criticalextraction can be found, for example, in van Bommel, M. J., and de Haan,A. B. J. Materials Sci. 29 (1994) 943-948, Francis, A. W. J. Phys. Chem.58 (1954) 1099-1114 and McHugh, M. A., and Krukonis, V. J. SupercriticalFluid Extraction: Principles and Practice. Stoneham, Mass.,Butterworth-Heinemann, 1986.

According to one embodiment the aerogel can be characterized by at leastone of the following features: a) comprising crystalline zirconiaparticles having an average primary particle size in a range from 10 nmto 50 nm; b) content of crystalline zirconia particles: at least about85 mol.-%; c) having a BET surface area in the range of 100 m²/g to 300m²/g; d) having an organic content of at least 3 wt.-%; e) x, y, zdimension: at least about 5 mm; f) showing a hysteresis loop (especiallyin a p/p₀ range of 0.70 to 0.95) when the N₂ adsorption/desorptionbehaviour is analysed; g) showing a type H1 hysteresis loop (accordingto IUPAC classification); h) showing a N₂ adsorption of isotherm type IV(according to IUPAC classification).

A combination of features (a) and (f) or (c) and (f) or (a), (c) and (h)is sometimes preferred.

Crack-Free, Calcined Metal Oxide Articles

Crack-free, calcined metal oxide articles can have x, y, and zdimensions of at least 3 mm (in some embodiments, at least 5 mm, 10 mm,15 mm, 20 mm, or even at least 25 mm) and a density of at least 30 (insome embodiments, at least 35, 40, 50, 95; in a range from 30 to 95)percent of theoretical density, and an average connected pore size in arange from 10 nm to 100 nm (in some embodiments, from 10 nm to 60 nm, 10nm to 50 nm, 10 nm to 40 nm, or even from 10 nm to 30 nm), wherein atleast 70 (in some embodiments, at least 75, 80, 85, 90, 95, 96, 97, 98,or even at least 99; in a range from 70 to 99, 75 to 99, 80 to 99, oreven 85 to 99) mole percent of the metal oxide is crystalline ZrO₂, andwherein the crystalline ZrO₂ has an average grain size less than 100 nm(in some embodiments, in a range from 20 nm to 100 nm, 30 nm to 100 nm,or even 30 nm to 70 nm).

Optionally, crack-free calcined metal oxide articles described hereincomprise at least one of Y₂O₃ (e.g., in a range from 1 to 15, 1 to 5, 6to 9, 3.5 to 4.5 or even 7 to 8) mole percent of the crystalline metaloxide is Y₂O₃), La₂O₃ (e.g., up to 5 mole percent La₂O₃), Al₂O₃ (e.g.,up to 0.5 mole percent Al₂O₃). One exemplary crack-free calcined metaloxide article comprises in a range from 1 to 5 mole percent of thecrystalline metal oxide is Y₂O₃, and in a range from 0 to 2 mole percentcrystalline metal oxide is La₂O₃, and in a range from 93 to 99 molepercent of the crystalline metal oxide is ZrO₂. Another exemplarycrack-free calcined metal oxide article comprises in a range from 6 to 9mole percent of the crystalline metal oxide is Y₂O₃, and in a range from0 to 2 mole percent crystalline metal oxide is La₂O₃, and in a rangefrom 89 to 94 mole percent of the crystalline metal oxide is ZrO₂.Another exemplary crack-free calcined metal oxide article comprises in arange from 3.5 to 4.5 mole percent Y₂O₃, and in a range from 0 to 2 molepercent crystalline metal oxide is La₂O₃, and in a range from 93.3 to96.5 mole percent ZrO₂. Another exemplary crack-free calcined metaloxide article comprises in a range from 7 to 8 mole percent Y₂O₃, and ina range from 0 to 2 mole percent crystalline metal oxide is La₂O₃, andin a range from 90 to 93 mole percent ZrO₂.

In some embodiments, the crack-free, calcined metal oxide article has asulfate equivalent less than 5 ppm and/or a chloride equivalent lessthan 5 ppm. The raw material used to prepare the zirconia sol oftencontains chloride and sulfate impurities. Several thousand ppm by weightof these ions can be present in the calcined metal oxide article. If notremoved these impurities can volatilize at the temperatures used forsintering and become entrapped in the sintered body as pores. Thechloride and sulfate impurities can be removed prior to sintering, forexample, by infiltrating the calcined body with a solution of ammonia inwater, allowing it to sand overnight, then exchanging the ammoniasolution with water several times. During this treatment ammonia reactswith the chloride and sulfate impurities to form soluble ammonia salts.These are removed by diffusion into the water. It is also possible toremove these impurities by adjusting the heating profile so thatsufficient volatilization occurs in the thermal treatment used to formthe calcined article.

Crack-free, calcined metal oxide articles described herein can be madeby a method comprising heating an aerogel described herein for a timeand at at least one temperature sufficient to provide the crack-free,calcined metal oxide article. Typically, the aerogel is slowly heated atrates in the range from 5° C./hr to 20° C./hr to 600° C. to removeorganics. Slow heating below 600° C. is typically necessary to volatizethe organics without cracking the body, for example, because ofnonuniform shrinkage or internal pressure of the volatile products.Thermogravimetric analysis and dilatometry can be used to track theweight loss and shrinkage which occurs at different heating rates. Theheating rates in different temperature ranges can then be adjusted tomaintain a slow and near constant rate of weight loss and shrinkageuntil the organics are removed. Careful control of the organic removalis critical to obtain crack-free bodies. Once the organic is removed thetemperature can be raised at a faster rate (e.g., 100° C./hr to 600°C./hr) to a temperature in the range from 800° C. to 1100° C. and heldat that temperature up to 5 hours. At these temperatures the strength ofthe material increases by additional sintering, but an open porestructure is retained. When an ion-exchange treatment is used to removechloride and sulfate impurities, the temperature and time used forheating the calcined body is such that it is strong enough to resist thecapillary forces associated with infiltration of an ammonia solution.Typically this requires a relative density above 40% of theoretical(preferably above 45%). For articles that are to be milled, having thetemperature too high and/or time too long can make milling difficult. Insome cases it may be convenient to conduct the organic burnoutseparately; however, in that case care may be necessary to preventabsorption of moisture from the atmosphere prior to the highertemperature treatment. The aerogel can be quite fragile after heating tojust 600° C., and nonuniform absorption of moisture can result incracking.

Useful embodiments of crack-free, calcined metal oxide articlesdescribed herein include mill blocks, including dental mill blocks.

Crack-Free, Crystalline Metal Oxide Articles

Crack-free, crystalline metal oxide articles described herein have an x,y, and z dimensions of at least 3 mm (in some embodiments, at least 5mm, 10 mm, 15 mm, 20 mm, or even 25 mm) and a density of at least 98.5(in some embodiments, 99, 99.5, 99.9, or even at least 99.99) percent oftheoretical density, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂, and wherein the ZrO₂ has an average grain size lessthan 400 nanometers (in some embodiments, less than 300 nanometers, 200nanometers, 150 nanometers, 100 nanometers, or even less than 80nanometers).

Optionally, crack-free, crystalline metal oxide articles describedherein comprise at least one of Y₂O₃ (e.g., in a range from 1 to 15, 1to 5, 6 to 9, 3.5 to 4.5 or even 7 to 8) mole percent of the crystallinemetal oxide is Y₂O₃), La₂O₃ (e.g., up to 5 mole percent La₂O₃), Al₂O₃(e.g., up to 0.5 mole percent Al₂O₃). One exemplary crack-free,crystalline metal oxide article comprises in a range from 1 to 5 molepercent of the crystalline metal oxide is Y₂O₃, 0 to 2 mole percent ofthe crystalline metal oxide is La₂O₃ and in a range from 93 to 97 molepercent of the crystalline metal oxide is ZrO₂. This general compositionhas been observed to yield a combination of high biaxial flexurestrength and good optical transmittance. Another exemplary crack-free;crystalline metal oxide article comprises in a range from 6 to 9 molepercent of the crystalline metal oxide is Y₂O₃, 0 to 2 mole percent ofthe crystalline metal oxide is La₂O₃, and in a range from 89 to 94 molepercent of the crystalline metal oxide is ZrO₂. This general compositionrange has been observed to yield a combination of good biaxial flexurestrength and high optical transmittance. Another exemplary crack-free,crystalline metal oxide article comprises in a range from 3.5 to 4.5mole percent Y₂O₃, 0-2 mole percent of the crystalline metal oxide isLa₂O₃ and in a range from 93.5 to 96.5 mole percent ZrO₂. This generalcomposition has been observed to yield a combination of especially highbiaxial flexure strength and good optical transmittance. Anotherexemplary crack-free, crystalline metal oxide article comprises in arange from 7 to 8 mole percent Y₂O₃, 0 to 2 mole percent of thecrystalline metal oxide is La₂O_(3.)and in a range from 90 to 93 molepercent ZrO₂. This general composition range has been observed acombination of good biaxial flexure strength and especially high opticaltransmittance. The lower yttria compositions are therefore believed tobe more desirable where high strength is required and moderate opticaltransmittance is sufficient. The higher yttria compositions aretherefore believed to be more desirable where high optical transmittanceis required and moderate strength is sufficient.

In another aspect, the present disclosure provides a method of makingcrack-free, crystalline metal oxide articles described herein, themethod comprising heating a crack-free, calcined metal oxide articledescribed herein for a time and at at least one temperature sufficientto provide the crack-free, crystalline metal oxide article. Typically,the heating is conducted at at least one temperature in a range from1000° C. to 1400° C. (in some embodiments, from 1000° C. to 1400° C.,1000° C. to 1350° C., or even 1200° C. to 1300° C.). Typically, all theheating at or above 1000° C. is conducted in less than 24 hours;typically in a range from about 2 to about 24 hours. Typically, all theheating at or above 1000° C. is conducted at less than 1.25 atm. ofpressure. Typically, the heating rate to temperature is in a range from50° C./hr. to 600° C./hr. Heating can be conducted in conventionalfurnaces, preferably those with programmable heating capabilities. Thematerial to be heated can be placed, for example, in an aluminacrucible.

In some embodiments of the crack-free, crystalline metal oxide article,the ZrO₂ is all cubic ZrO₂. In some embodiments, the ZrO₂ is alltetragonal. In some embodiments, the zirconia is a mixture of tetragonaland cubic. Although not wanting to be bound by theory, based on theequilibrium phase diagram for ZrO₂ and Y₂O₃, mixtures of the cubic andtetragonal phases would be expected when the Y₂O₃ content is in therange from 2 to 8 mole percent and the material is sintered in the rangefrom about 1200° C. to about 1250° C.

Embodiments with about 3.5 to 4.5 mole percent Y₂O₃ with a mixture oftetragonal and some cubic structure exhibit an exceptional combinationof strength and optical transmittance. When sintered at 1250° C. for 2hr the average grain size in one instance was 156 nm. Surprisingly onlythe tetragonal crystal structure was observed even though a mixture oftetragonal and cubic crystals would be expected in this compositionrange. When these materials were held at the sintering temperature for aprolonged time the grain size increased to 168 nm, a mixture oftetragonal and cubic crystalline phases was formed, and the goodtransmittance of the material was substantially reduced. In a similarmanner, if the sintering temperature was raised to about 1500° C. andheld for 2 hours, the grain size increased to 444 nm, additional cubicphase was formed, and the good optical transmittance was furtherdegraded. It appears that maintaining the single phase tetragonalcrystal structure and grain size of this composition below 175 nm ishelpful for good optical transmission of this composition. A possibleexplanation for the nonequilibrium structures is that the chemicalelements are very uniformly distributed in the nanoparticles making upthe initial sol and aerogel. These are able to sinter to full densitybefore the elements can segregate into equilibrium structures whichwould normally be present.

Embodiments containing about 7 to 8 mole percent Y₂O₃, with a mixture ofcubic and some tetragonal structure, exhibit the best transmittance, andmay be particularly useful in applications where lower strength can betolerated. Although a mixture of tetragonal and cubic phases would beexpected for this composition the material was entirely cubic. This issurprising as it would be expected that compositions composed entirelyof the cubic phase would exhibit the best transmission as there would beno tetragonal phase to scatter light.

In some embodiments, the crack-free, crystalline metal oxide article hasa total transmittance of at least 65% at a thickness of 1 mm asdetermined by the procedure under the heading “Total Transmittance,Diffuse Transmittance, Haze” in the Example section below.

In some embodiments, the crack-free, crystalline metal oxide article iscolorless in visual appearance.

In some embodiments, the crack-free, crystalline metal oxide article isopalescent in visual appearance.

In some embodiments, the crack-free, crystalline metal oxide article hasan average biaxial flexural strength of at least 300 MPa (in someembodiments, at least 500 MPa, 750 MPa, 1000 MPa, or even at least 1300MPa).

Exemplary uses of crack-free, crystalline metal oxide articles describedherein include optical windows, implants (e.g. tooth implants,artificial hip, and knee joints), and dental articles (e.g.,restoratives (see, for example, FIG. 4 showing crown 400 with veneer 404and coping 404, wither of which or both can comprising crack-free,crystalline metal oxide described herein), replacements, inlays, onlays,veneers, full and partial crowns, bridges, implants, implant abutments,copings, anterior fillings, posterior fillings, and cavity liner, andbridge frameworks) and orthodontic appliances (e.g., brackets, buccaltubes, cleats, and buttons). These articles could be milled, forexample, from calcined oxide articles (mill-blocks) using computerizedmilling machines. Heating to temperatures near 1250° C. could completesintering to full density.

According to one embodiment the crystalline metal oxide article can becharacterized by the following features:

(a) showing a N2 adsorption of isotherm type IV according to IUPACclassification;(b) showing a hysteresis loop when the N2 absorption/desorptionbehaviour is analysed;(c) showing a N2 adsorption of isotherm type IV according to IUPACclassification and a hysteresis loop,(d) showing a N2 adsorption of type IV with a hysteresis loop of type H1according to IUPAC classification,(e) showing a N2 adsorption of type IV with a hysterese loop of type H1according to IUPAC classification in a p/p0 range of 0.70 to 0.95;(f) average connected pore diameter: from about 10 to about 100 nm orfrom about 10 to about 70 nm or from about 10 to about or from about 10to about 50 nm or from about 15 to about 40;(g) average grain size: less than about 100 nm or less than about 80 nmor less than about 60 nm or from about 10 to about 100 or from about 15to about 60 nm;(h) BET surface: from about 10 to about 200 m²/g or from about 15 toabout 100 m²/g or from about 16 to about 60 m²/g;(i) x, y, z dimension: at least about 5 mm or at least about 10 or atleast about 20 mm.

A combination of the following features was found to be particularlybeneficial: (a) and (h), or (a) and (b) and (h), or (c) and (g), or (c),(e), (g) and (h).

Surprisingly it was found that material showing a N2 adsorption ofisotherm type IV (according to IUPAC classification) and/or a hysteresisloop (especially in a p/p0 range of 0.70 to 0.95) are particularlysuitable.

According to one embodiment, the crystalline metal oxide article can beobtained by a process comprising the steps of

-   -   providing a zirconia sol comprising crystalline metal oxide        particles and a solvent,    -   optionally concentrating the zirconia sol to provide a        concentrated zirconia sol,    -   mixing the sol with a polymerizable organic matrix (e.g. adding        a reactive surface modifier to the zirconia sol and optionally        an initiator being able to polymerizable surface-modified        particles of the zirconia sol);    -   optionally casting the zirconia sol into a mould to provide a        casted zirconia sol,    -   curing the polymerizable organic matrix of the zirconia sol to        form a gel (sometimes also referred to as gelation step),    -   removing the solvent from the gel (e.g. by first removing water,        if present, from the gel via a solvent exchange process to        provide an at least partially de-watered gel; followed by a        further extraction step where the remaining solvent is extracted        e.g. via super critical extraction) to provide the aerogel,    -   optionally cutting the aerogel into smaller pieces,    -   heat-treating the aerogel to obtain a machinable article.

According to a more specific embodiment, the crystalline metal oxidearticle can be obtained by a process comprising the steps of:

-   -   providing a zirconia sol comprising crystalline metal oxide        particles,    -   optionally concentrating the zirconia sol to provide a        concentrated zirconia sol,    -   adding a radically reactive surface modifier to the zirconia sol        to provide surface-modified particles of the zirconia sol,    -   adding a radical initiator to the radically polymerizable        surface-modified particles of the zirconia sol,    -   optionally casting the zirconia sol into a mould to provide a        casted zirconia sol,    -   curing the radically polymerizable surface-modified particles of        the zirconia sol to form a gel,    -   optionally removing water, if present, from the gel via a        solvent exchange to provide an at least partially de-watered        gel,    -   extracting solvent, if present, from the gel via super critical        extraction to provide the aerogel,    -   optionally cutting the aerogel block into smaller pieces,    -   heat-treating the aerogel to obtain a machinable article.

Exemplary Embodiments

1A. An aerogel (in some embodiments, a monolithic aerogel (i.e., havingx, y, and z dimensions of at least 1 mm (in some embodiments, at least1.5 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, or even at least10 mm)) comprising organic material and crystalline metal oxideparticles, wherein the crystalline metal oxide particles are in a rangefrom 3 to 20 volume percent, based on the total volume of the aerogel,wherein at least 70 mole percent of the crystalline metal oxide is ZrO₂.

2A. The aerogel of Embodiment 1A, wherein the crystalline metal oxideparticles have an average primary particle size in a range from 2nanometers to 50 nanometers.

3A. The aerogel of any preceding Embodiment, wherein the crystallinemetal oxide particles are in a range from 1 to 15 (in some embodiments,1 to 9, 1 to 5, 6 to 9, 3.5 to 4.5 or even 7 to 8) mole percent of thecrystalline metal oxide is Y₂O₃.

4A. The aerogel of any of Embodiments 1A to 3A, wherein the crystallinemetal oxide particles further comprise at least one of Y₂O₃ or La₂O₃.

5A. The aerogel of any preceding Embodiment, wherein the crystallinemetal oxide particles further comprise at least one of CeO₂, Pr₂O_(3.)Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃,Yb₂O₃, Fe₂O₃, MnO₂, CO₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, or Lu₂O₃.

6A. The aerogel of any of Embodiments 1A to 3A, wherein the crystallinemetal oxide particles further comprise at least one of Fe₂O₃, MnO₂,Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, Er₂O₃, Pr₂O₃, Eu₂O₃, Dy₂O₃, Sm₂O₃,or CeO₂.

7A. The aerogel of any preceding Embodiment, wherein the crystallinemetal oxide particles further comprise Al₂O₃.

8A. The aerogel of any preceding Embodiment, wherein the organic contentis at least 3 percent by weight, based on the total weight of theaerogel.

9A. The aerogel of any preceding Embodiment having a surface area in arange from 100 m²/g to 300 m²/g.

10A. The aerogel of any preceding Embodiment having an average connectedpore size in a range from 10 nm to 20 nm.

11A. The aerogel of any preceding Embodiment, wherein the crystallinemetal oxide particles comprise a first plurality of particles, and asecond, different plurality of particles.

1B. A method of making the aerogel (in some embodiments, a monolithicaerogel (i.e., having x, y, and z dimensions of at least 1 mm (in someembodiments, at least 1.5 mm, 2 mm, 3 mm, 4 mm, 5 mm, or even at least10 mm)) comprising organic material and crystalline metal oxideparticles, wherein the crystalline metal oxide particles are in a rangefrom 3 to 20 volume percent, based on the total volume of the aerogel,wherein at least 70 mole percent of the crystalline metal oxide is ZrO₂,the method comprising:

providing a first zirconia sol comprising crystalline metal oxideparticles having an average primary particle size of not greater than 50nanometers, wherein at least 70 mole percent of the crystalline metaloxide is ZrO₂;

optionally concentrating the first zirconia sol to provide aconcentrated zirconia sol;

adding a radically reactive surface modifier to the zirconia sol toprovide a radically polymerizable surface-modified zirconia sol;

adding a radical initiator to the radically polymerizablesurface-modified zirconia sol;

heating at at least one temperature for a time sufficient to polymerizethe radically surface-modified zirconia sol comprising the radicalinitiator to form a gel;

optionally removing water, if present, from the gel via alcohol exchangeto provide an at least partially de-watered gel; and extracting alcohol,if present, from the gel via super critical extraction to provide theaerogel.

2B. The method of Embodiment 1B further comprising adding a radicallyreactive co-monomer to the concentrated zirconia sol.

1C. A method of making a crack-free, calcined metal oxide article havingx, y, and z dimensions of at least 5 mm, a density in as range from 30to 95 percent of theoretical density, and an average connected pore sizein a range from 10 nm to 100 nm, wherein at least 70 mole percent of themetal oxide is crystalline ZrO₂, and wherein the crystalline ZrO₂ has anaverage grain size less than 100 nm, the method comprising heating themonolithic aerogel of any of Embodiments 1A to 11A for a time and at atleast one temperature sufficient to provide the crack-free, calcinedmetal oxide article.

2C. The method of Embodiment 1C further comprising chemically treatingthe calcined metal oxide article to remove volatile ions.\

3C. The method of Embodiment 1C further comprising chemically treatingthe calcined metal oxide article to at least one remove Cl ions or SO₄ions.

1D. A crack-free, calcined metal oxide article having x, y, and zdimensions of at least 5 mm, a density in a range from 30 to 95 percentof theoretical density, and an average connected pore size in a rangefrom 10 nm to 100 nm, wherein at least 70 mole percent of the metaloxide is crystalline ZrO₂, and wherein the crystalline ZrO₂ has anaverage grain size less than 100 nm.

2D. The crack-free, calcined metal oxide of Embodiment 1D, wherein thecrack-free, calcined metal oxide article has x, y, and z dimensions ofat least 10 mm.

3D. The crack-free, calcined metal oxide of either Embodiment 1D or 2D,wherein at least 75 mole percent of the crystalline metal oxide presentin the crack-free, calcined metal oxide article is crystalline ZrO₂.

4D. The crack-free, calcined metal oxide of any of Embodiments 1D to 3D,wherein the crystalline metal oxide comprises in a range from 1 to 15(in some embodiments, 1 to 9, 1 to 5, 6 to 9 3.5 to 4.5, or even 7 to 8)mole percent of the crystalline metal oxide is Y₂O₃.

5D. The crack-free, calcined metal oxide of any of Embodiments 1D to 4D,wherein the crystalline metal oxide further comprises at least one ofY₂O₃ or La₂O₃.

6D. The crack-free, calcined metal oxide of any of Embodiments 1D to 5D,wherein the crystalline metal oxide further comprises at least one ofCeO₂, Pr₂O_(3.) Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃,Er₂O₃, Tm₂O₃, Yb₂O₃, Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃,or Lu₂O₃.

7D. The crack-free, calcined metal oxide of any of Embodiments 1D to 5D,wherein the crystalline metal oxide further comprises at least one ofFe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, Er₂O₃, Pr₂O₃, Eu₂O₃,Dy₂O₃, Sm₂O₃, or CeO₂.

8D. The crack-free, calcined metal oxide of Embodiments 1D to 7D,wherein the crystalline metal oxide further comprises Al₂O₃.

9D. The crack-free, calcined metal oxide of any of Embodiments 1D to 8Dhaving a sulfate equivalent less than 5 ppm and a chloride equivalentless than 5 ppm.

10D. The crack-free, calcined metal oxide of any of Embodiments 1D to 9Dwhich is a mill block.

1E. A method of making a crack-free, crystalline metal oxide articlehaving an x, y, and z dimensions of at least 3 mm and a density of atleast 98.5 (in some embodiments, at least 99, 99.5, 99.9, or even atleast 99.99) percent of theoretical density, wherein at least 70 molepercent of the crystalline metal oxide is ZrO₂, and wherein the ZrO₂ hasan average grain size less than 400 nanometers (in some embodiments,less than 300 nanometers, 200 nanometers, 150 nanometers, 100nanometers, or even less than 80 nanometers), the method comprisingheating a crack-free, calcined metal oxide article of any of Embodiments1D to 10D for a time and at at least one temperature sufficient toprovide the crack-free, crystalline metal oxide article.

2E. The method of Embodiment 1E, wherein the heating is conducted at atleast one temperature in a range from 1150° C. to 1300° C.

3E. The method of either Embodiment 1E or 2E, wherein all the heating isconducted in less than 24 hours.

4E. The method of any of Embodiments 1E to 3E, wherein all the heatingis conducted at less than 1.25 atm. of pressure.

5E. The method of any of Embodiments 1E to 4E, wherein the ZrO₂ presentin the crack-free, crystalline metal oxide article is all cubic ZrO₂.

6E. The method of any of Embodiments 1E to 4E, wherein the ZrO₂ presentin the crack-free, crystalline metal oxide article comprises alltetragonal ZrO₂.

7E. The method of any of Embodiments 1E to 4E, wherein the ZrO₂ presentin the crack-free, crystalline metal oxide article comprises cubic andtetragonal ZrO₂.

8E. The method of any of Embodiments 1E to 7E, wherein the crack-free,crystalline metal oxide article has a total transmittance of at least65% at a thickness of 1 mm.

9E. The method of any of Embodiments 1E to 8E, wherein the crack-free,crystalline metal oxide article is colorless.

10E. The method of any of Embodiments 1E to 8E, wherein the crack-free,crystalline metal oxide article is opalescent.

11E. The method of any of Embodiments 1E to 10E, wherein the crack-free,crystalline metal oxide article has a biaxial flexural strength of atleast 300 MPa (in some embodiments, in a range from 300 MPa to 1300MPa).

12E. The method of any of Embodiments 1E to 11E, wherein the crack-free,crystalline metal oxide article is a dental article.

13E. The method of Embodiment 12E, wherein the dental article isselected from the group consisting of restoratives, replacements,inlays, onlays, veneers, full and partial crowns, bridges, implants,implant abutments, copings, anterior fillings, posterior fillings, andcavity liner, and bridge frameworks.

14E. The method of any of Embodiments 1E to 13E, wherein the crack-free,crystalline metal oxide article is an orthodontic appliance.

15E. The method of Embodiment 14E, wherein the orthodontic appliance isselected from the group consisting of brackets, buccal tubes, cleats,and buttons.

1F. A crack-free, crystalline metal oxide article having an x, y, and zdimensions of at least 3 mm and a density of at least 98.5 (in someembodiments, at least 99, 99.5, 99.9, or even at least 99.99) percent oftheoretical density, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂, and wherein the ZrO₂ has an average grain size in arange from 75 nanometers to 400 nanometers.

2F. The crack-free, crystalline metal oxide article of Embodiment 1F,wherein the crystalline metal oxide comprises in a range from 1 to 15mole percent (in some embodiments 1 to 9 mole percent) Y₂O₃.

3F. The crack-free, crystalline metal oxide article of either Embodiment1F or 2F, wherein the crystalline metal oxide further comprises La₂O₃.

4F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 3F, wherein the crystalline metal oxide furthercomprises at least one of CeO₂, Pr₂O_(3.) Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃,Gd₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Fe₂O₃, MnO₂, CO₂O₃,Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, or Lu₂O₃.

5F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 3F, wherein the crystalline metal oxide furthercomprises at least one of Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃,Ga₂O₃, Er₂O₃, Pr₂O₃, Eu₂O₃, Dy₂O₃, Sm₂O₃, or CeO₂.

6F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 5F, wherein the crystalline metal oxide furthercomprises Al₂O₃.

7F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 6F, wherein the ZrO₂ is all cubic ZrO₂.

8F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 7F, wherein the ZrO₂ is all tetragonal ZrO₂.

9F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 7F, wherein the ZrO₂ comprises cubic and tetragonalZrO₂.

10F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 9F having a total transmittance of at least 65% at athickness of 1 mm.

11F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 10F that is colorless.

12F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 10F that is opalescent.

13F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 12F that passes the Hydrolytic Stability Test.

14F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 13F that is a dental article.

15F. The crack-free, crystalline metal oxide article of Embodiment 14F,wherein the dental article is selected from the group consisting ofrestoratives, replacements, inlays, onlays, veneers, full and partialcrowns, bridges, implants, implant abutments, copings, anteriorfillings, posterior fillings, and cavity liner, and bridge frameworks.

16F. The crack-free, crystalline metal oxide article of any ofEmbodiments 1F to 13F that is an orthodontic appliance.

17F. The crack-free, crystalline metal oxide article of Embodiment 16F,wherein the orthodontic appliance is selected from the group consistingof brackets, buccal tubes, cleats, and buttons.

1G. A crack-free, crystalline metal oxide article having an x, y, and zdimensions of at least 3 mm and a density of at least 99.5 (in someembodiments, at least 99, 99.5, 99.9, or even at least 99.99) percent oftheoretical density, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂, wherein in range from 1 to 5 mole percent (in someembodiments 3.5 to 4.5 mole percent) of the crystalline metal oxide isY₂O₃, and wherein the ZrO₂ has an average grain size 75 nanometers to175 nanometers (in some embodiments, in a range from 100 nanometers to165 nanometers).

2G. The crack-free, crystalline metal oxide article of Embodiment 1G,wherein the crystalline metal oxide further comprises La₂O₃.

3G. The crack-free, crystalline metal oxide article of either Embodiment1G or 2G, wherein the crystalline metal oxide further comprises at leastone of CeO₂, Pr₂O_(3.) Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃,Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₂,Ga₂O₃, or Lu₂O₃.

4G. The crack-free, crystalline metal oxide article of either Embodiment1G or 2G, wherein the crystalline metal oxide further comprises at leastone of Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, Er₂O₃, Pr₂O₃,Eu₂O₃, Dy₂O₃, Sm₂O₃, or CeO₂.

5G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 4G, wherein the crystalline metal oxide furthercomprises Al₂O₃.

6G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 5G, wherein the ZrO₂ is all tetragonal ZrO₂.

7G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 5G, wherein the ZrO₂ comprises cubic and tetragonalZrO₂.

8G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 7G having a total transmittance of at least 65% at athickness of 1 mm.

9G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 7G that is colorless.

10G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 9G that is opalescent.

11G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 10G that passes the Hydrolytic Stability Test.

12G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 11G that is a dental article.

13G. The crack-free, crystalline metal oxide article of Embodiment 12G,wherein the dental article is selected from the group consisting ofrestoratives, replacements, inlays, onlays, veneers, full and partialcrowns, bridges, implants, implant abutments, copings, anteriorfillings, posterior fillings, and cavity liner, and bridge frameworks.

14G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 11G that is an orthodontic appliance.

15G. The crack-free, crystalline metal oxide article of Embodiment 14G,wherein the orthodontic appliance is selected from the group consistingof brackets, buccal tubes, cleats, and buttons.

16G. The crack-free, crystalline metal oxide article of any ofEmbodiments 1G to 15G, wherein the crack-free, crystalline metal oxidearticle is composed entirely of the tetragonal crystal structure.

1H. A method of making a crack-free, crystalline metal oxide article ofany of Embodiments 1G to 12G the method comprising heating a crack-free,calcined metal oxide article for a time and at at least one temperaturesufficient to provide the crack-free, crystalline metal oxide article,the crack-free, calcined metal oxide article having an x, y, and zdimensions of at least 3 mm, a density in a range from 30 to 95 percentof theoretical density, and an average connected pore size in a rangefrom 10 nm to 100 nm, wherein at least 70 mole percent of the metaloxide is crystalline ZrO₂, wherein at least 70 mole percent of thecrystalline metal oxide is ZrO₂, wherein in range from 1 to 5 molepercent (in some embodiments 3.5 to 4.5 mole percent) of the crystallinemetal oxide is Y₂O₃, and wherein the crystalline ZrO₂ has an averagegrain size less than 100 nm.

2H. The method of Embodiment 1H, wherein the heating is conducted at atleast one temperature in a range from 1150° C. to 1300° C.

3H. The method of either Embodiment 1H or 2H, wherein all the heating isconducted in less than 24 hours.

4H. The method of any of Embodiments 1H to 3H, wherein all the heatingis conducted at less than 1.25 atm. of pressure.

5H. The method of any of Embodiments 1H to 4H, wherein the crack-free,calcined metal oxide article has x, y, and z dimensions of at least 10mm.

6H. The method of any of Embodiments 1H to 5H, wherein at least 75 molepercent of the crystalline metal oxide present in the crack-free,calcined metal oxide article is crystalline ZrO₂.

7H. The method of any of Embodiments 1H to 6H, wherein the crack-free,calcined metal oxide article crystalline metal oxide further comprisesLa₂O₃.

8H. The method of any of Embodiments 1H to 7H, wherein the crack-free,calcined metal oxide article crystalline metal oxide further comprisesat least one of CeO₂, Pr₂O_(3.) Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃,Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃,NiO, CuO, Bi₂O₃, Ga₂O₃, or Lu₂O₃.

9H. The method of any of Embodiments 1H to 7H, wherein the crack-free,calcined metal oxide article crystalline metal oxide further comprisesat least one of Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃,Er₂O₃, Pr₂O₃, Eu₂O₃, Dy₂O₃, Sm₂O₃, or CeO₂.

10H. The method of Embodiments 1H to 9H, wherein the crystalline metaloxide further comprises Al₂O₃.

11H. The method of any of Embodiments 1H to 10H, wherein the crack-free,calcined metal oxide has a sulfate equivalent less than 5 ppm and achloride equivalent less than 5 ppm.

12H. The method of any of Embodiments 1H to 11H, wherein the crack-freecalcined metal oxide article is a mill block.

1I. A crack-free, crystalline metal oxide article having an x, y, and zdimensions of at least 3 mm and a density of at least 98.5 (in someembodiments, 99, 99.5, 99.9, or at least at least 99.99) percent oftheoretical density, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂, wherein in range from 6 to 9 mole percent (in someembodiments 7 to 8 mole percent) of the crystalline metal oxide is Y₂O₃,and wherein the ZrO₂ has an average grain size in a range from 100nanometers to 400 nanometers (in some embodiments, in a range from 200nanometers to 300 nanometers).

2I. The crack-free, crystalline metal oxide article of Embodiment 1I,wherein the crystalline metal oxide further comprises La₂O₃.

3I. The crack-free, crystalline metal oxide article of either Embodiment1I or 2I, wherein the crystalline metal oxide further comprises at leastone of CeO₂, Pr₂O_(3.) Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃,Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃,Ga₂O₃, or Lu₂O₃.

4I. The crack-free, crystalline metal oxide article of either Embodiment1I or 2I, wherein the crystalline metal oxide further comprises at leastone of Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, Er₂O₃, Pr₂O₃,Eu₂O₃, Dy₂O₃, Sm₂O₃, or CeO₂.

5I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 4I, wherein the crystalline metal oxide furthercomprises Al₂O₃.

6I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 5I, wherein the ZrO₂ is all cubic ZrO₂.

7I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 5I, wherein the ZrO₂ comprises cubic and tetragonalZrO₂.

8I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 7I having a total transmittance of at least 65% at athickness of 1 mm.

9I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 8I that is colorless.

10I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 8I that is opalescent.

11I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 10I that passes the Hydrolytic Stability Test.

12I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 11I that is a dental article.

13I. The crack-free, crystalline metal oxide article of Embodiment 12I,wherein the dental article is selected from the group consisting ofrestoratives, replacements, inlays, onlays, veneers, full and partialcrowns, bridges, implants, implant abutments, copings, anteriorfillings, posterior fillings, and cavity liner, and bridge frameworks.

14I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 11I that is an orthodontic appliance.

15I. The crack-free, crystalline metal oxide article of Embodiment 14I,wherein the orthodontic appliance is selected from the group consistingof brackets, buccal tubes, cleats, and buttons.

16I. The crack-free, crystalline metal oxide article of any ofEmbodiments 1I to 15I, wherein the crack-free, crystalline metal oxidearticle contains less than 8 mole percent of Y2O3 and is composedentirely of the cubic crystal structure.

1J. A method of making a crack-free, crystalline metal oxide article ofany of Embodiments 1J to 15J, the method comprising heating acrack-free, calcined metal oxide article for a time and at at least onetemperature sufficient to provide the crack-free, crystalline metaloxide article, the crack-free, calcined metal oxide article having x, y,and z dimensions of at least 3 mm, a density in a range from 30 to 95percent of theoretical density, and an average connected pore size in arange from 10 nm to 100 nm, wherein at least 70 mole percent of themetal oxide is crystalline ZrO₂, wherein at least 70 mole percent of thecrystalline metal oxide is ZrO₂, wherein in range from 6 to 9 molepercent (in some embodiments, 7 to 8 mole percent) of the crystallinemetal oxide is Y₂O₃, and wherein the crystalline ZrO₂ has an averagegrain size less than 100 nm.

2J. The method of Embodiment 1J, wherein the heating is conducted at atleast one temperature in a range from 1150° C. to 1300° C.

3J. The method of either Embodiment 1J or 2J, wherein all the heating isconducted in less than 24 hours.

4J. The method of any of Embodiments 1J to 3J, wherein all the heatingis conducted at less than 1.25 atm. of pressure.

5J. The method of any of Embodiments 1J to 4J, wherein the crack-free,calcined metal oxide article has x, y, and z dimensions of at least 10mm.

6J. The method of any of Embodiments 1J to 5J, wherein at least 75 molepercent of the crystalline metal oxide present in the crack-free,calcined metal oxide article is crystalline ZrO₂.

7J. The method of any of Embodiments 1J to 6J, wherein the crack-free,calcined metal oxide article further comprises La₂O₃.

8J. The method of any of Embodiments 1 J to 7J, wherein the crack-free,calcined metal oxide article further comprises at least one of CeO₂,Pr₂O_(3.) Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃,Tm₂O₃, Yb₂O₃, Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, orLu₂O₃.

9J. The method of any of Embodiments 1J to 8J, wherein the crack-free,calcined metal oxide article further comprises at least one of Fe₂O₃,MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, Er₂O₃, Pr₂O₃, Eu₂O₃, Dy₂O₃,Sm₂O₃, or CeO₂.

10J. The method of Embodiments 1J to 9J, wherein the wherein thecrack-free, calcined metal oxide article further comprises furthercomprises Al₂O₃.

11J. The method of any of Embodiments 1J to 10J, wherein the crack-free,calcined metal oxide article further comprises has a sulfate equivalentless than 5 ppm and a chloride equivalent less than 5 ppm.

12J. The method of any of Embodiments 1J to 11J, wherein the crack-free,calcined metal oxide article is a mill block.

1K. A method of making a crack-free, crystalline metal oxide articlehaving an x, y, and z dimensions of at least 3 mm and a density of atleast 98.5 (in some embodiments, at least 99, 99.5, 99.9, or even atleast 99.99) percent of theoretical density, wherein at least 70 molepercent of the crystalline metal oxide is ZrO₂, and wherein the ZrO₂ hasan average grain size less than 300 nanometers, the method comprisingpressureless heating in air a crack-free, calcined metal oxide articlehaving x, y, and z dimensions of at least 5 mm, a density of at least 30percent of theoretical density, wherein at least 70 mole percent of themetal oxide is crystalline ZrO₂, and wherein the crystalline ZrO₂ has anaverage grain size less than 100 nm for a time and at at least onetemperature sufficient to provide the crack-free, crystalline metaloxide article, wherein the method is conducted at no greater than 1400°C.

2K. The method of Embodiment 1K, wherein the heating is conducted at atleast one temperature in a range from 1000° C. to 1400° C. (in someembodiments, 1000° C. to 1350° C., or even 1200° C. to 1300° C.).

3K. The method of either Embodiment 1K or 2K, wherein all the heating isconducted in less than 24 hours.

4K. The method of any of Embodiments 1K to 3K, wherein the ZrO₂ presentin the crack-free, crystalline metal oxide article is all cubic ZrO₂.

5K. The method of any of Embodiments 1K to 3K, wherein the ZrO₂ presentin the crack-free, crystalline metal oxide article comprises alltetragonal ZrO₂.

6K. The method of any of Embodiments 1K to 3K, wherein the ZrO₂ presentin the crack-free, crystalline metal oxide article comprises cubic andtetragonal ZrO₂.

7K. The method of any of Embodiments 1K to 6K, wherein the crack-free,calcined metal oxide article further comprises La₂O₃.

8K. The method of any of Embodiments 1K to 7K, wherein the crack-free,calcined metal oxide article comprises at least one of CeO₂, Pr₂O_(3.)Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₁, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃,Yb₂O₃, Fe₂O₃, MnO₂, Co₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, or Lu₂O₃.

9K. The method of any of Embodiments 1K to 7K, wherein the crack-free,calcined metal oxide article comprises at least one of Fe₂O₃, MnO₂,CO₂O₃, Cr₂O₃, NiO, CuO, Bi₂O₃, Ga₂O₃, Er₂O₃ Pr₂O₃, Eu₂O₃, Dy₂O₃, Sm₂O₃,or CeO₂.

10K. The method of Embodiments 1K to 9K, wherein the crack-free,calcined metal oxide article further comprises Al₂O₃.

11K. The method of any of Embodiments 1K to 10K, wherein the crack-free,crystalline metal oxide article has a total transmittance of at least65% at a thickness of 1 mm.

12K. The method of any of Embodiments 1K to 11K, wherein the crack-free,crystalline metal oxide article is colorless.

13K. The method of any of Embodiments 1K to 11K, wherein the crack-free,crystalline metal oxide article is opalescent.

14K. The method of any of Embodiments 1K to 13K, wherein the crack-free,crystalline metal oxide article has a biaxial flexural strength of atleast 300 MPa (in some embodiments, in a range from 300 MPa to 1300MPa).

15K. The method of any of Embodiments 1K to 14K, wherein the crack-free,crystalline metal oxide article is a dental article.

16K. The method of Embodiment 15K, wherein the dental article isselected from the group consisting of restoratives, replacements,inlays, onlays, veneers, full and partial crowns, bridges, implants,implant abutments, copings, anterior fillings, posterior fillings, andcavity liner, and bridge frameworks.

17K. The method of any of Embodiments 1K to 16K, wherein the crack-free,crystalline metal oxide article is an orthodontic appliance.

18K. The method of Embodiment 17K, wherein the orthodontic appliance isselected from the group consisting of brackets, buccal tubes, cleats,and buttons.

Advantages and embodiments of this invention are further illustrated bythe following examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention. All parts andpercentages are by weight unless otherwise indicated. In the followingexamples, “mol %” means mole percent.

Test Methods X-Ray Diffraction Analysis (XRD)

Samples of sintered bodies were examined without any further changes.Reflection geometry data were collected in the form of a survey scan byuse of a diffractometer (obtained under the trade designation “BRUKER D8ADVANCE DIFFRACTOMETER” from Bruker Corporation, Madison, Wis.) withcopper K_(α) radiation, and detector registry of the scatteredradiation. The diffractometer was fitted with variable incident beamslits and fixed diffracted beam slits. The survey scan was conducted inthe coupled continuous mode from 10 to 80 degrees (2θ) using a 0.015degree step size and 2 second dwell time. The x-ray generator settingsof 40 kilovolts and 40 milliamps were employed.

The observed diffraction peaks were identified by comparison to thereference diffraction patterns contained within the International Centerfor Diffraction Data (ICDD) powder diffraction database (sets 1-47,ICDD, Newton Square, Pa.) and attributed to cubic (C), tetragonal (T),or monoclinic (M) phases of zirconia. The (111) peak was used for thecubic phase, the (101) peak was used for the tetragonal phase, and the(−111) and (111) peaks were used for the monoclinic phase. Because ofthe small crystalline size of the particles as prepared in the sol, the(111) peak for the cubic phase and the (101) peak for the tetragonalphase could not be separated. The phases are reported together as the C(111)/T (101) peak. The amounts of each zirconia phase were evaluated ona relative basis and the form of zirconia having the most intensediffraction peak was assigned the relative intensity value of 100. Thestrongest line of the remaining crystalline zirconia phase was scaledrelative to the most intense line and given a value between 1 and 100.

Peak widths for the observed diffraction maxima due to corundum weremeasured by profile fitting. The relationship between mean corundum peakwidths and corundum peak position (2θ) was determined by fitting apolynomial to these data to produce a continuous function used toevaluate the instrumental breadth at any peak position within thecorundum testing range. Peak widths for the observed diffraction maximadue to zirconia were measured by profile fitting the observeddiffraction peaks.

A Pearson VII peak shape model with K_(α1) and K_(α2) wavelengthcomponents accounted for, and linear background model were employed inall cases. Widths were found as the peak full width at half maximum(FWHM) having units of degrees. The profile fitting was accomplished byuse of the capabilities of the JADE diffraction software suite.

Sample peaks were corrected for instrumental broadening by interpolationof instrumental breadth values from corundum instrument calibration andcorrected peak widths converted to units of radians. The Scherrerequation was used to calculate the primary crystal size.

Crystallite Size (D)=Kλ/β(cos θ)

In the Scherrer equation, K is the form factor (here 0.9), λ is thewavelength (1.540598 Å), β is the calculated peak width after correctionfor instrumental broadening (in radians), and θ equals half the peakposition (scattering angle). β is equal to [calculated peakFWHM−instrumental breadth] (converted to radians) where FWHM is fullwidth at half maximum.

The weighted average of the cubic/tetragonal (C/T) and monoclinic phases(M) were calculated using the following equation.

Weighted average=[(% C/T)(C/T size)+(% M)(M size)]/100

In this equation, % C/T equals the percent crystallinity contributed bythe cubic and tetragonal crystallite content of the ZrO₂ particles; C/Tsize equals the size of the cubic and tetragonal crystallites; % Mequals the percent crystallinity contributed by the monocliniccrystallite content of the ZrO₂ particles; and M size equals the size ofthe monoclinic crystallites.

Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP)

Inductively Coupled Plasma Atomic Emission Spectroscopy was used toanalyze the zirconia-based sol samples for the lanthanide and yttriumelement concentration. Liquid samples were aspirated into a hightemperature argon plasma where desolvation, dissociation, atomization,and excitation occur. Each element has a well established andcharacteristic wavelengths associated with emission from an excitedstate. The intensity of the emission is typically proportional to theconcentration of the element. The concentration of the element can becalculated by comparing the intensity of the emission with that ofstandards of known concentration.

The zirconia-based sols (0.2 to 0.3 gram) were accurately weighed into acentrifuge tube. Deionized water (40 ml) and hydrochloric acid (2 mlconcentrated hydrochloric acid (37-38 percent; obtained from EMDChemicals, Gibbstown, N.J. under trade designation EMD OMNITRACE)) wasadded. The solutions were then diluted to a total of 50 grams withdeionized water. Duplicates of each sample were prepared. Two blankscontaining just the hydrochloric acid and water were also prepared.Further dilutions were prepared as necessary to bring the concentrationof the samples within the calibration range. The samples and blanks wereanalyzed on an Inductively Coupled Plasma optical emission spectrometer(obtained under the trade designation “PERKIN ELMER OPTIMA 4300” fromPerkin Elmer, Shelton, Conn.). The instrument was calibrated usingmulti-element standards. The standards, which were obtained fromsolutions that are available from High Purity Standards, Charleston,S.C., had concentrations of 0.2 ppm, 0.5 ppm, and 1.5 ppm (microgram permilliliter). The results were normalized to the amount of zirconia inthe starting zirconia-based sol.

Photon Correlation Spectroscopy (PCS)

Particle size measurements were made using a light scattering particlesizer equipped with a red laser having a 633 nm wavelength of light(obtained under the trade designation “ZETA SIZER—NANO SERIES, MODELZEN3600” from Malvern Instruments Inc., Westborough, Mass.). Each samplewas analyzed in a one centimeter square polystyrene sample cuvette. Thesample cuvette was filled with about 1 gram of deionized water, and thena few drops (about 0.1 gram) of the zirconia-based sol were added. Thecomposition (e.g., sample) within each sample cuvette was mixed bydrawing the composition, into a clean pipette and discharging thecomposition back into the sample cuvette several times. The samplecuvette was then placed in the instrument and equilibrated at 25° C. Theinstrument parameters were set as follows: dispersant refractive index1.330, dispersant viscosity 1.0019 MPa-second, material refractive index2.10, and material absorption value 0.10 units. The automaticsize-measurement procedure was then run. The instrument automaticallyadjusted the laser-beam position and attenuator setting to obtain thebest measurement of particle size.

The light scattering particle sizer illuminated the sample with a laserand analyzed the intensity fluctuations of the light scattered from theparticles at an angle of 173 degrees. The method of Photon CorrelationSpectroscopy (PCS) was used by the instrument to calculate the particlesize. PCS uses the fluctuating light intensity to measure Brownianmotion of the particles in the liquid. The particle size is thencalculated to be the diameter of sphere that moves at the measuredspeed.

The intensity of the light scattered by the particle is proportional tothe sixth power of the particle diameter. The Z-average size or cumulantmean is a mean calculated from the intensity distribution and thecalculation is based on assumptions that the particles are mono-modal,mono-disperse, and spherical. Related functions calculated from thefluctuating light intensity are the Intensity Distribution and its mean.The mean of the Intensity Distribution is calculated based on theassumption that the particles are spherical. Both the Z-average size andthe Intensity Distribution mean are more sensitive to larger particlesthan smaller ones.

The Volume Distribution gives the percentage of the total volume ofparticles corresponding to particles in a given size range. Thevolume-average size is the size of a particle that corresponds to themean of the Volume Distribution. Since the volume of a particle isproportional to the third power of the diameter, this distribution isless sensitive to larger particles than the Z-average size. Thus, thevolume-average will typically be a smaller value than the Z-averagesize.

Field Emission Scanning Electron Microscopy (FESEM)

Samples were prepared by depositing a thin layer of Au—Pd to make thesamples conductive. The microscope used was a field emission scanningelectron microscope obtained under the trade designation “HITACHIS-4700” from Hitachi Ltd., Maidenhead, UK. Images (i.e., electronmicrographs) were obtained while operating at 3.0 or 5.0 kilovolts andwith a magnification of 30,000 or 70,000 times.

Line Intercept Analysis

FESEM micrographs with 70,000 times magnification were used for grainsize measurement. Three or four micrographs taken from different areasof the sintered body were used for each sample. Ten horizontal lines,which were spaced at roughly equal intervals across the height of eachmicrograph, were drawn. The number of grain boundary intercepts observedon each line were counted and used to calculate the average distancebetween intercepts. The average distance for each line was multiplied by1.56 to determine the grain size and this value was averaged over allthe lines for all micrographs of each sample.

Method for Measuring N2 Sorption Isotherms, BET Surface Area, PoreVolume, Average Connected Pore Size and Porosity

The samples were run on either on a QUANTACHROME AUTOSORB-1 BETAnalyzer” (Quantachrome Instruments, Boynton Beach, Fla.) or aBELSORP-mini instrument (BEL Japan Inc., Osaka, Japan). The samples wereweighed and outgassed at 200° C. for two days then subjected to a N2sorption process with an appropriate number and distribution ofmeasurement points, e.g. 55 adsorb points and 20 desorb points from aP/P_(o) range 1×10⁶ to 1 and back to 0.05 giving full isotherms. Thespecific surface area S was calculated by the BET method (Detailsregarding calculation see Autosorb-1 Operating Manual Ver. 1.51 IV.Theory and Discussion, Quantachrome Instruments, Inc.). The total porevolume V_(liq) is derived from the amount of vapor adsorbed at arelative pressure close to unity (P/P_(o) closest to 1), by assumingthat the pores are then filled with liquid adsorbate (Details regardingcalculation see Autosorb-1 Operating Manual Ver. 1.51 IV. Theory andDiscussion, Quantachrome Instruments, Inc.). The average pore diameter(d) is calculated from the surface area (S) and the total pore volume

${\left( V_{liq} \right)\text{:}\mspace{14mu} d} = \frac{4\; {Vliq}}{S}$

Method for Measuring Weight Percent Solids

The weight percent solids were determined by drying a sample weighing3-6 grams at 120° C. for 30 minutes. The percent solids can becalculated from the weight of the wet sample (i.e., weight beforedrying, weight_(wet)) and the weight of the dry sample (i.e., weightafter drying, weight_(dry)) using the following equation.

Wt-% solids=100(weight_(dry))/weight_(wet)

Method for Measuring Oxide Content of a Solid

The oxide content of a sol sample is determined by measuring the percentsolids content as described in the “Method for Measuring Weight PercentSolids” then measuring the oxide content of those solids as described inthis section.

The oxide content of a solid was measured via thermal gravimetricanalysis (obtained under the trade designation “TGA Q500” from TAInstruments, New Castle, Del.). The solids (about 50 mg) were loadedinto the TGA and the temperature was taken to 900° C. The oxide contentof the solid is equal to the residual weight after heating to 900° C.

Method for Measuring Biaxial Flexure Strength

The strength of various zirconia bodies was measured using the biaxialflexure strength.

Samples were circular sintered wafers, roughly 12 mm in diameter and 1.5mm thick. The wafers were ground to different thickness on a polishingwheel using a 45 micrometer metal bonded diamond disc (identified asPart No: 156145 from Buehler, Lake Bluff, Ill.), followed by 30micrometer and 9 micrometer diamond lapping film (obtained under thetrade designation “3M DIAMOND LAPPING FILM 668X” from 3M Company, St.Paul, Minn.) and finally 3 micrometer diamond suspension (obtained underthe trade designation “METADI DIAMOND SUSPENSION” from Buehler) on apolishing cloth (obtained under the trade designation “TEXMET POLISHINGCLOTH” front Buehler). A minimum of 4 samples were measured to determinethe average strength.

The polished side of each wafer was centered on a support consisting ofthree steel balls, 3 mm diameter, spaced at 120° intervals, on a circlewith a diameter of 8 mm. The support and wafer were placed in a fixturewith a vertical punch that rested on the center of the upper, unpolishedside of the wafer. The punch diameter in contact with the wafer was 1.8mm. The fixture was loaded in a universal test machine (identified as“Series 1101” from Applied Test Systems, Inc., Butler, Pa.). The punchwas pushed into the wafer at a rate of 0.2 mm per min. until the waferfractured. The load at fracture was recorded. The strength value wascalculated from the following formula:

S=−0.2387P(X−Y)/d ²

Where:

P=load at fracture in Newtons

X=(1+v)ln(r ₂ /r ₃)²+[(1−v)/2](r ₂ /r ₃)²

Y=(1+v)[1+ln(r ₁ /r ₃)²]+(1−v)(r ₁ /r ₃)²

In which:v=Poisson's Ratio (assumed a value of 0.23)r₁=the radius of the support circle in mmr₂=the radius of the upper punch contact in mmr₃=the radius of the sample wafer in mmd=the thickness of the sample wafer in mm

Volume Percent Metal Oxide

The volume percent of oxide present in an aerogel or a calcined metaloxide was determined by back-calculation using shrinkage data andassuming that the final sintered body was a 1 cm cube, 100% dense. Thetotal volume of the aerogel or calcined metal oxide is then(Vt)=[1/(1−S)]³, where S is the fractional shrinkage from the aerogel orcalcined state to the final sintered material. The volume of metal oxideis the volume of the sintered cube (V)=1. The percent metal oxide (Vol%)=(1/V_(t))100.

Method for Measuring Archimedes' Density

The density of the sintered material was measured by an Archimedestechnique. The measurements were made on a precision balance (identifiedas “AE 160” from Mettler Instrument Corp., Hightstown, N.J.) using adensity determination kit (identified as “ME 33360” from MettlerInstrument Corp.). In this procedure the sample was first weighed in air(A), then immersed in water (B). The water was distilled and deionized.One drop of a wetting agent (obtained under trade designation“TERGITOL-TMN-6” from Dow Chemical Co., Danbury, Conn.) was added to 250ml of water. The density was calculated using the formula ρ=(A/(A−B))ρ₀,where ρ₀ is the density of water.

The relative density can be calculated by reference to the theoreticaldensity (ρ_(t)) of the material, ρ_(rel)=(ρ/ρ_(t))100.

Total Transmittance, Diffuse Transmittance, Haze

The samples were measured using a spectrophotometer (obtained under thetrade designation “PERKIN ELMER LAMBDA 1050” from Perkin ElmerCorporation, Waltham, Mass.) fitted with a integrating sphere accessory.This sphere was 150 mm (6 inches) in diameter and complied with ASTMmethods E903, D1003, E308, et al. as published in “ASTM Standards onColor and Appearance Measurement”, Third Edition, ASTM, 1991. The valuesof Total (TLT) and Diffuse (DLT) Light Transmittance corresponding toCommission Internationale de L'Eclairage (CIE) Light Source C with awavelength range between 380 nm and 780 nm were calculated from thesum-product of the TLT and DLT transmitted spectra using the CIEweighting table. The recorded TLT and DLT spectra used for thecalculations were measured from 250 nm to 830 nm and are shown in FIGS.2 and 3.

% Haze was calculated according to ASTM D1003 (CIE Source C) as follows:

% Haze=(% DLTs/% TLTs)*100,

where TLTs is the TLT of the sample, DLTs is the DLT of the sample.

A small spot accessory was used with center focus so that the area ofthe sample illuminated at the front sample port was about the same asthe area illuminated at the rear sample port (where white plate or lighttrap were used to record T100 and T0 spectra, respectively). The testparameters were as follows:

Scan Speed: 102 nm/min (approximately)UV-Vis Integration: 0.56 ms/pt

Data Interval: 1 nm Slit Width: 5 nm Mode: ° A) Transmission (Total andDiffuse) Hydrolytic Stability Test

The hydrolytic stability of some Examples was tested generally accordingto ISO 13356:2008, entitled “Implants for surgery—Ceramic MaterialsBased On Yttria-Stabilized Tetragonal Zirconia (Y-TZP)”, chapter 4.8(2008).

More specifically, sintered samples were placed in an autoclave andexposed to saturated steam at 135° C. under a pressure of 0.2 MPa for 5hours.

After the 5 hour exposure to saturated steam at 135° C. under a pressureof 0.2 MPa, the crystal phases of the sample surface were measured withx-ray diffraction equipment with a Bragg-Brentano geometry (obtainedunder the trade designation “BRUKER D8 DISCOVER” from Bruker AXS GmbH,Karlsruhe, Germany) with quantitative phase analysis based on theRietveld method (software obtained under the trade designation “BRUKERTOPAS” from Bruker AXS GmbH, Karlsruhe, Germany) to determine the amountof monoclinic phase.

To pass this Hydrolytic Stability Test, not more than 25% monoclinicphase is permitted after being subjected to the 5 hour exposure tosaturated steam at 135° C. under a pressure of 0.2 MPa.

Materials Used

TABLE 1 Material name or abbreviation Description MEEAA2-(2-(2-Methoxyethoxy) Ethoxy) Acetic Acid obtained from AldrichChemical Company, Milwaukee, WI Zirconium acetate An aqueous solution ofzirconium acetate containing nominally 16.3 weight percent Zr obtainedfrom Magnesium Elektron, Inc., Flemington, NJ. The aqueous solution wasexposed to an ion exchange resin (obtained under the trade designation“AMBERLYTE IR 120” from Rohm and Haas Company, Philadelphia, PA) beforeuse (oxide content 21.85 wt.%) DI water De-ionized water Yttrium acetateYttrium (III) acetate tetrahydrate obtained from AMR Technologies Inc.,Toronto, Canada (oxide content 33.4 wt. %) 1-Methoxy-2- An alcoholobtained from Aldrich Chemical Company propanol 2-Hydroxyethyl Anacrylate monomer obtained from Aldrich Chemical Company methacrylate(HEMA) Triethylamine A base obtained from Aldrich Chemical CompanyLanthanum Lathanum (III) acetate hydrate obtained from Alfa Aesar, WardHill, MA Acetate (oxide content 45.5 wt. %) Acrylamide Acrylamideobtained from Alfa Aesar 1-vinyl-2- 1-vinyl-2-pyrrolidione obtained fromAlfa Aesar pyrrolidione 2,2′-Azobis(2-2,2′-Azobis(2-methylbutyronitrile), obtained from E. I. du Pont demethylbutyronitrile), Nemours and Company, Wilmington, DE under thetrade designation (“VAZO 67”) “VAZO 67” Ethoxylated EthoxylatedPentaerythritol Tetraacrylate, obtained from Sartomer PentaerythritolCompany Inc., Exton, PA, under the trade designation “SR454”Tetraacrylate (“SR454”) Ethoxylated Ethoxylated PentaerythritolTetraacrylate, obtained from Sartomer Pentaerythritol Company Inc.,under the trade designation “SR494” Tetraacrylate (“SR494”) PolyethyleneGlycol Polyethylene Glycol (400) dimethacrylate, obtained from Sartomer(400) dimethacrylate Company Inc., under the trade designation “SR603”(“SR603”) Ethoxylated (9) Ethoxylated (9) Trimethylolpropane TriacrylateTrimethylolpropane Obtained from Sartomer Company Inc., under the tradedesignation Triacrylate “SR502” (“SR502”) Ethoxylated (15) Ethoxylated(15) Trimethylolpropane Triacrylate Trimethylolpropane Obtained fromSartomer Company Inc., under the trade designation Triacrylate “SR9035”(“SR9035”) Butyl Acrylate Butyl Acrylate obtained from Alfa Aesar

Preparation of ZrO₂ (95.7 mol %)/Y₂O₃ (2.3 mol %)/La₂O₃ (2 mol %) Sol(Sol T1)

Sol compositions are reported in mole percent inorganic oxide. Thefollowing hydrothermal reactor was used for preparing Sol T1 and allother sols in this application.

The hydrothermal reactor was prepared from 15 meters of stainless steelbraided smooth tube hose (0.64 cm inside diameter, 0.17 cm thick wall;obtained under the trade designation “DUPONT T62 CHEMFLUOR PTFE” fromSaint-Gobain Performance Plastics, Beaverton, Mich.). This tube wasimmersed in a bath of peanut oil heated to the desired temperature.Following the reactor tube, a coil of an additional 3 meters ofstainless steel braided smooth tube hose (“DUPONT T62 CHEMFLUOR PTFE”;0.64 cm I.D., 0.17 cm thick wall) plus 3 meters of 0.64 cmstainless-steel tubing with a diameter of 0.64 cm and wall thickness of0.089 cm that was immersed in an ice-water bath to cool the material anda backpressure regulator valve was used to maintain an exit pressure of2.76 MPa.

A precursor solution was prepared by combining the zirconium acetatesolution (2,000 grams) with DI water (1000 grams). Yttrium acetate (57.6grams) was added while mixing until full dissolution. Lanthanum acetate(53.1 grams) and D.I water (600 grams) were added and mixed until fullydissolved. The solids content of the resulting solutions was measuredgravimetrically (120° C./hr. forced air oven) to be 21.9 wt. %. D.I.water (567 grams) was added to adjust the final concentration to 19 wt.%. This procedure was repeated four times of give a total of about17,100 grams of precursor material. The resulting solution was pumped ata rate of 11.48 ml/min. through the hydrothermal reactor. Thetemperature was 225° C. and the average residence time was 42 minutes. Aclear and stable zirconia sol was obtained.

Preparation of ZrO₂ (88 Mol %)/Y₂O₂ (12 Mol %) Sol (Sol C2)

A precursor solution was prepared by combining the zirconium acetatesolution (2,000 grams) with DI water (2000 grams). Yttrium acetate(326.8 grams) was added while mixing. The solids content of theresulting solutions was measured gravimetrically (120° C./hr. forced airoven) to be 22.2 wt. %. D.I. water (728 grams) was added to adjust thefinal concentration to 19 wt. %. This procedure was repeated three timesto produce a total of about 15,100 grams of precursor solution. Theresulting solution was pumped at a rate of 11.48 ml/min. through thehydrothermal reactor. The temperature was 225° C. and the averageresidence time was 42 minutes. A clear and stable zirconia sol wasobtained.

Table 2 (below) is a summary of the compositions and the processconditions used for other sols produced in a similar manner to Sol T1.

TABLE 2 ZrO₂, Y₂0₃, La₂O₃, Residence Temperature, Sol mol % mol % mol %time, min. ° C. T1 95.7 2.3 2.0 42 225 T2 95.7 2.3 2.0 42 225 C1 88 12 042 225 C2 88 12 0 42 225 C3 88 12 0 42 225 C4 88 12 0 42 225 B1 95 5 042 225 S1 97.7 2.3 0 42 215 S2 97.7 2.3 0 42 215 S3 97.7 2.3 0 42 215 S497.7 2.3 0 42 215 A1 95 3.0 2.0 42 225

Sol Concentration and Diafiltration

The resulting sols were concentrated (20-35 wt. % solids) first viaultrafiltration using a membrane cartridge (obtained under the tradedesignation “M21S-100-01P” from Spectrum Laboratories Inc., RanchoDominguez, Calif.), and then via constant volume diafiltration using thesame membrane cartridge. The resulting sol was then further concentratedvia rotary evaporation.

The diafiltration process resulted in some loss of yttrium and lanthanumfrom the zirconia sol. ICP was used to determine the following data.

A sol prepared at 97.5/2.3/2 ZrO₂:Y₂O₃:La₂O₃ resulted in a sol with thefollowing composition 96.6/2.2/1.3 ZrO₂:Y₂O₃:La₂O₃.

A sol prepared with an 88/12 ZrO₂/Y₂O₃ composition resulted in a solwith the following composition 90.7/9.3 ZrO₂:Y₂O₃.

A sol prepared with a 97.7/2.3 ZrO₂/Y₂O₃ composition resulted in a solwith the following composition 97.7/2.3 ZrO₂:Y₂O₃.

A sol prepared with a 95/5 ZrO₂/Y₂O₃ composition resulted in a sol withthe following composition 95.6/4.4 ZrO₂:Y₂O₃.

Comparative Example A

A partially sintered zirconia-based material (60 mm zirconia block;obtained under the trade designation “LAVA” 3M ESPE, St. Paul, Minn.)was removed from a 3-unit frame (obtained under the trade designation“LAVA” from 3M ESPE). The cylindrical block was diced into wafers 1-2millimeter in thickness with a low speed diamond saw using de-ionizedwater as a lubricant. The wafers were dried at 60° C. and then sinteredin a rapid temperature furnace (obtained from CM Furnaces Inc.,Bloomfield, N.J.) by heating at a rate of 7.5° C./minute to 1500° C.;holding at 1500° C. for 2 hours; and cooling at 10° C./minute to 20° C.

Sintered wafers were ground to different thicknesses on a polishingwheel using a 45 micrometer metal bonded diamond disc (obtained as PartNo: 156145 from Buehler), followed by 30 micrometer and 9 micrometerdiamond lapping film (“3M DIAMOND LAPPING FILM 668X”) and finally 3micrometer diamond suspension (“METADI DIAMOND SUSPENSION”) on apolishing cloth (“TEXMET POLISHING CLOTH”). Each wafer was mounted in alapping fixture (obtained as Model 150 from South Bay Technology, Inc.,Temple City, Calif.) during grinding and polishing to maintain flat andparallel faces. Wafers were bonded to the lapping fixture using ahot-melt adhesive (obtained under trade designation “QUICKSTICK 135”from South Bay Technology, Inc., Temple City, Calif.). One side of eachwafer was ground and polished, then the wafer was remounted and theother side was ground and polished. Polishing to finer finishes hadnegligible impact on the measured transmission. Wafers with thefollowing thickness values in millimeters were prepared; 1.00, 0.85,0.60, 0.50, 0.45, and 0.38.

The optical density (OD) of each wafer was measured on a densitometer(obtained under the trade designation “TD504” from Macbeth, Newburgh,N.Y.). The total transmission (T) was calculated using the formula:

OD=log₁₀(1/T)

The total transmission was then plotted as a function of wafer thickness(t) and found to form a near linear plot with the following equation:

T=−24.249+52.704

This equation was used to calculate the Lava reference transmission atany desired thickness in the range.

Example wafers were ground and polished and the total transmissionmeasured following the same procedures used for the Lava wafers. Theratio of this value to the Lava value (T/T_(L)) calculated for the samethickness was used for comparative purposes.

To measure the total transmittance, diffuse transmittance, and haze ofComparative Example A, a partially sintered zirconia-based material(block 60 mm; “LAVA”) was removed from a 3-unit frame (“LAVA”). A wafer2 mm thick was diced from the block with a low speed diamond saw usingde-ionized water as a lubricant. The wafer was dried at 90-125° C.

The wafer was set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a rapid temperature furnace (obtained from CMFurnaces Inc.): i—heat from 20° C. to 1500° C. at 450° C./hr. rate;ii—hold at 1500° C. for 2 hours; and iii—cool down from 1500° C. to 20°C. at 600° C./hr. rate.

The sintered wafer was polished on both faces using polishing equipmentcomprised of an electrically driven head obtained under the tradedesignation (“VECTOR POWER HEAD” from Buehler) and a grinder-polisher(obtained under the trade designation “BETA GRINDER-POLISHER” fromBuehler). First the sample was ground flat using a 45 micrometer metalbonded diamond disc (identified as Part No: 156145 from Buehler). Then30 micrometer diamond lapping film (“3M DIAMOND LAPPING FILM 668X”) wasused until the majority of the 45 micrometer scratches were removed.Then 9 micrometer diamond lapping film (“3M DIAMOND LAPPING FILM 668X”)was used until the majority of the 30 micrometer scratches were removed.Next the sample was polished using 3 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 9 micrometer scratches were removed.Finally the sample was polished using 0.25 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 3 micrometer scratches were removed.The wafer was mounted in a lapping fixture (Model 150, South BayTechnology, Inc.) during grinding and polishing to maintain flat andparallel faces. The wafer \vas bonded to the lapping fixture using ahot-melt adhesive (“QUICKSTICK 135”). One side of the wafer was groundand polished, then the wafer was remounted and the other side was groundand polished.

The total transmittance was 27.9%, the diffuse transmittance was 27.7%,and the haze was 99.4%, measured using the spectrophotometer proceduredescribed earlier. The TLT and DLT spectra are designated in FIGS. 2 and3 as 1000 and 1100, respectively. The sample thickness was 0.99 mm.

To measure the biaxial flexural strength of Comparative Example A, apartially sintered zirconia-based material (block 60 mm; “LAVA”) wasremoved from a 3-unit frame (“LAVA”). The diameter of the cylindricalblock was ground down to a diameter of 18 mm using a 45 micrometer metalbonded diamond disc (identified as Part No: 156145 from Buehler). Thereduced block was diced into wafers 1.1 mm in thickness with a low speeddiamond saw using de-ionized water as a lubricant. The wafers were driedat 60° C.

The wafers were set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a rapid temperature furnace (CM Furnaces Inc.):i—heat from 20° C. to 1500° C. at 450° C./hr. rate; ii—hold at 1500° C.for 2 hours; and iii—cool down from 1500° C. to 20° C. at 600° C./hr.rate.

This produced wafers of about the same dimensions as the materialsdescribed in the examples. It was then possible to use the same fixturegeometry for comparative strength measurements.

The wafers were polished on one face using a 12 open face lappingmachine (obtained under the trade designation “LAPMASTER” from LapmasterInternational Limited, Mt. Prospect, Ill.) for all but the finalpolishing step. The samples were all adhered to a sample plate and werethen ground flat using 20 micrometer diamond tile (obtained under thetrade designation “3M TRIZACT DIAMOND TILE” from 3M Company) at a speedof 30 rpm. The abrasive was then switched to 9 micrometer diamond tile(obtained under the trade designation “3M TRIZACT DIAMOND TILE” from 3MCompany) and grinding continued at 30 rpm until the majority of the 20micrometer scratches were removed. The abrasive was then switched to 3micrometer diamond tile (obtained under the trade designation “3MTRIZACT DIAMOND TILE” from 3M Company) and grinding continued at 30 rpmuntil the majority of the 9 micrometer scratches were removed. The finalpolish was done using polishing equipment comprised of an electricallydriven head (“VECTOR POWER HEAD”) and a grinder-polisher (“BETAGRINDER-POLISHER”) and 3 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the scratches were removed.

The biaxial flexure strength of 15 wafers was measured using theprocedure described earlier. The average value was 1101 MPa.

Example 1 and 2

To prepare Example 1, Sol S1 was diafiltered and concentrated asdescribed above for Sol T1. The resulting sol was 55.4 wt. % ZrO₂/Y₂O₃and 5.65 wt % acetic acid. The sol (200 grams) was charged to a 500 mlround bottom (RB) flask. Ethanol (60.6 grams), acrylic acid (11.5grams), and HEMA (5.9 grams) were added to the flask.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.6 gram) was added andthe contents stirred for 4 hours. The contents of the flask were thenpurged with N₂ gas for 6 minutes. The sample (translucent and lowviscosity) was charged to cylindrical containers (29 mm diameter). Eachcontainer was about 18 mlin volume and each was sealed on both ends(very little air gap was left between the top and liquid). The sampleswere allowed to stand about 1 hour then placed in an oven to cure (50°C., 4 hours). This results in a clear translucent blue gel. The gel wasremoved from the container and placed in a 473 mlwide mouth jar. The jarwas filled with ethanol (denatured). The sample was soaked for 24 hrthen the ethanol was replaced with fresh ethanol. The sample was soakedfor 24 hr then the ethanol was replaced with a third batch of freshethanol. The sample was allowed to soak until the supercriticalextraction was done. The above manipulations were done minimizing theamount of time the gel was exposed to the air.

To prepare Example 2, Sol S2 was diafiltered and concentrated asdescribed above for Sol T1. The resulting sol was 55.8 wt % ZrO₂/Y₂O₃and about 5.5 wt % acetic acid. The sol (195.6 grams) was charged to a500 ml RB flask. Ethanol (60.6 grams), acrylic acid (11.5 grams) andHEMA (5.9 grams) were added to the flask.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.60 gram) was added andthe contents stirred for 4 hours. The contents of the flask were thenpurged with N2 gas for 12.5 minutes). The sample (translucent and lowviscosity) was charged to cylindrical containers (29 mm diameter). Eachcontainer was about 18 ml in volume and each was sealed on both ends(very little air gap was left between the top and liquid). The sampleswere allowed to stand about 1 hour then placed in an oven to cure (50°C., 4 hours). This results in a clear translucent blue gel. The gel wasremoved from the container and placed in a 473 ml wide mouth jar. Thejar was filled with ethanol (denatured). The sample was soaked for 24hours then the ethanol was replaced with fresh ethanol. The sample wassoaked for 24 hours then the ethanol was replaced with a third batch offresh ethanol. The sample was allowed to soak until the supercriticalextraction was done. The above manipulations were done minimizing theamount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Examples 1 and 2 were removed separately fromthe ethanol bath, weighed, placed individually inside small canvaspouches, and then stored briefly in another ethanol bath. The wet weightof Example 1 was 19.9 grams. The wet weight of Example 2 was 23 grams.About 790 ml of 200-proof ethanol was added to the 10-l extractor of alaboratory-scale supercritical fluid extractor unit designed by andobtained from Thar Process, Inc., Pittsburgh, Pa. The canvas bagscontaining the wet zirconia-based gels were transferred from the ethanolbath into the 10-l extractor so that the wet gels were completelyimmersed in the liquid ethanol inside the jacketed extractor vessel,which was heated and maintained at 60° C. After the extractor vessel lidwas sealed in place, liquid carbon dioxide was pumped by a chilledpiston pump (setpoint: 12.5° C.) through a heat exchanger to heat theCO₂ to 60° C. and into the 10-L extractor vessel until an internalpressure of 13.3 MPa was reached. At these conditions, carbon dioxide issupercritical. Once the extractor operating conditions of 13.3 MPa and60° C. were met, a needle valve regulated the pressure inside theextractor vessel by opening and closing to allow the extractor effluentto pass through a porous 316L stainless steel frit (obtained from MottCorporation, New Britain, Conn., as Model #1100S-5.480 DIA-.062-10-A),then through a heat exchanger to cool the effluent to 30° C., andfinally into a 5-L cyclone separator vessel that was maintained at roomtemperature and pressure less than 5.5 MPa, where the extracted ethanoland gas-phase CO₂ were separated and collected throughout the extractioncycle for recycling and reuse. Supercritical carbon dioxide (scCO₂) waspumped continuously through the 10-L extractor vessel for 7 hours fromthe time the operating conditions were achieved. After the 7-hourextraction cycle, the extractor vessel was slowly vented into thecyclone separator over 16 hours from 13.3 MPa to atmospheric pressure at60° C. before the lid was opened and the dried canvas pouches containingthe aerogels were removed. The dry aerogels were removed from theircanvas pouches, weighed, and transferred into a 237 ml glass jar packedwith tissue paper for storage. The dry Example 1 aerogel wassemi-translucent with a bluish tint and weighed 10.4 grams,corresponding to an overall weight loss during the supercriticalextraction process of 47.7%. The dry Example 2 aerogel wassemi-translucent with a bluish tint and weighed 12.5 grams,corresponding to an overall weight loss during the supercriticalextraction process of 45.7%.

Organic Burnout and Pre-Sinter Process

The extracted aerogel sample of Examples 1 from above was removed fromits closed container and its weight, diameter and height were measured.The sample was set on a bed of zirconia beads in an unglazed porcelaincrucible, covered with an alumina fiberboard then fired in air accordingto the following schedule in a high temperature furnace (obtained underthe trade designation “THERMOLYNE TYPE 46200” from Thermo FischerScientific, Inc., Waltham, Mass.): i—heat from 20° C. to 225° C. at 18°C./hr. rate; ii—hold at 225° C. for 24 hours; iii—heat from 225° C. to400° C. at 6° C./hr. rate; iv—heat from 400° C. to 600° C. at 18° C./hr.rate; and v—cool down from 600° C. to 20° C. at 600° C./hr. rate.

The sample was set on an alumina fiberboard contained in an aluminacrucible, covered with an alumina crucible then fired in air accordingto the following schedule in a crucible furnace (Model 56724;“LINDBERG/BLUE M 1700° C.” from Thermo Fischer Scientific, Inc.): i—heatfrom 20° C. to 665° C. at 600° C./hr. rate; ii—heat from 665° C. to 800°C. at 120° C./hr rate; and iii—cool down from 800° C. to 20° C. at 600°C./hr. rate.

The sample was then set on a bed of zirconia beads in an aluminacrucible, covered with an alumina fiberboard then fired in air accordingto the following schedule in a crucible furnace (Model 56724;LINDBERG/BLUE M 1700° C.): i—heat from 20° C. to 665° C. at 600° C./hr.rate; ii—heat from 665° C. to 950° C. at 120° C./hr. rate; iii—hold at950° C. for 2 hours; and iv—cool down from 950° C. to 20° C. at 600°C./hr. rate.

The extracted aerogel of Example 2 and pre-sintered aerogel of Example 1samples were analyzed to determine the BET surface area, pore size andporosity. The extracted aerogel of Example 2 (which was crack free) hada surface area of 198 m²/g, total pore volume of 0.806 cm³/g and anaverage pore diameter of 163 Angstroms (A). The pre-sintered sample ofExample 1 had a surface area of 35 m²/g, total pore volume of 0.285cm³/g and an average pore diameter of 329 A.

Example 3

A 277 gram sample of Sol T1 (prepared and diafiltered and concentratedas described above, 29.5 wt. % oxide and 3.2 wt. % acetic acid) wascharged to 500 ml round-bottom (RB) flask. Water (127 grams) was removedvia rotary evaporation, resulting in a viscous somewhat dry material.Ethanol (45.5 grams), acrylic acid (8.6 grams) and 2-Hydroxyethylmethacrylate (HEMA) (4.4 grams) were added to the flask. The contentswere stirred for about 4 hours resulting is a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.45 gram) was added andthe contents stirred for 5 minutes. The contents of the flask were thenpurged with N2 gas for 4 minutes. The sample (translucent and lowviscosity) was charged to cylindrical containers (29 mm diameter). Eachcontainer was about 18 ml in volume and each was sealed on both ends(very little air gap was left between the top and liquid). The sampleswere allowed to stand for about 1 hour then placed in an oven to cure(50° C., 4 hours). This results in a clear translucent blue gel. The gelwas removed from the container and placed in a 473 ml wide mouth jar.The jar was filled with ethanol (denatured). The sample was soaked for24 hr then the ethanol was replaced with fresh ethanol. The sample wassoaked for 24 hours then the ethanol was replaced with a third batch offresh ethanol. The sample was allowed to soak until the supercriticalextraction was done. The above manipulations were done minimizing theamount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Example 3 were removed separately from theethanol bath, weighed, placed inside an individual, small canvas pouch,and then stored briefly in another ethanol bath before being loaded intothe 10-L extractor vessel. The wet weight of Example 3A was 20.3 grams.The wet weight of Example 3B was 21.5 grams. The wet weight of Example3C was 15.5 grams. The wet weight of Example 3D was 18.8 grams. For allsamples in Example 3, about 800 ml of 200-proof ethanol was added to the10-L extractor of a laboratory-scale supercritical fluid extractor unit.The canvas bags containing the wet zirconia-based gels were transferredfrom the ethanol bath into the 10-L extractor so that the wet gels werecompletely immersed in the liquid ethanol inside the jacketed extractorvessel, which was heated and maintained at 60° C. The Example 3A-3Dsamples were subjected to the same extraction process as described abovefor Examples 1 and 2. Afterwards, the dry aerogels were removed fromtheir canvas pouches, weighed, and transferred into individual 237 mlglass jars packed with tissue paper for storage. The dry Example 3Aaerogel was semi-translucent with a bluish tint and weighed 10.8 grams,corresponding to an overall weight loss during the supercriticalextraction process of 46.8%. The dry Example 3B aerogel wassemi-translucent with a bluish tint and weighed 11.3 grams,corresponding to an overall weight loss during the supercriticalextraction process of 47.4%. The dry Example 3C aerogel wassemi-translucent with a bluish tint and weighed 8.2 grams, correspondingto an overall weight loss during the supercritical extraction process of47.1%. The dry Example 3D aerogel was semi-translucent with a bluishtint and weighed 9.9 grams, corresponding to an overall weight lossduring the supercritical extraction process of 47.3%.

Organic Burnout and Pre-Sinter Process

The extracted aerogel samples of Examples 3A-3D from above were removedfrom their closed containers and the weight, diameter and height weremeasured. The samples were set on alumina fiberboard supports in anunglazed porcelain crucible, covered with an alumina fiberboard thenfired in a high temperature furnace (“THERMOLYNE TYPE 46200”) in airaccording to the following schedule: i—heat from 20° C. to 225° C. at18° C./hr. rate; ii—hold at 225° C. for 24 hours; iii—heat from 225° C.to 400° C. at 6° C./hr. rate; iv—heat from 400° C. to 600° C. at 18°C./hr. rate; and v—cool down from 600° C. to 20° C. at 600° C./hr. rate.

After firing (i.e., organic burnout) the samples were crack free. Thesamples were then set on an alumina fiberboard support in an aluminacrucible, covered with an alumina fiberboard then pre-sinter fired inair according to the following schedule in a crucible furnace (Model56724; “LINDBERG/BLUE M 1700° C.”): i—heat from 20° C. to 665° C. at600° C./hr. rate; ii—heat from 665° C. to 1090° C. at 120° C./hr. rate;and iii—cool down from 1090° C. to 20° C. at 600° C./hr. rate.

After pre-sinter firing the samples were crack free. The cylinders werediced into about 1 mm thick wafers or about 1.5 mm thick wafers. Thewafers were ion exchanged by first placing them in a 118 ml glass jarcontaining distilled water at a depth of about 2.5 cm and then vacuuminfiltrating. The water was replaced with about a 2.5 cm depth of 1.0NNH₄OH and the wafers were soaked overnight for at least 16 hours. TheNH₄OH was then poured off and the jar was filled with distilled water.The wafers were soaked in the distilled water for 1 hour. The water wasthen replaced with fresh distilled water. This step was repeated untilthe pH of the soak water was equal to that of fresh distilled water. Thewafers were then dried at 90-125° C. for a minimum of 1 hour. Theaerogel of Example 3A had 12.2 volume % of oxides while the pre-sintered(at 1090° C.) aerogel of Example 3A had 41.7 volume % of oxides. TheVolume percent oxide values were calculated using the method describedabove.

Sintering Process

The pre-sintered wafers from above were set on an alumina fiberboard inan alumina crucible, covered with an alumina fiberboard then sintered inair according to the following schedule in a crucible furnace (Model56724: “LINDBERG/BLUE M 1700° C.”): i—heat from 20° C. to 1090° C. at600° C./hr. rate; ii—heat from 1090° C. to 1210° C. at 120° C./hr. rate;iii—hold at 1210° C. for 12 hours; and iv—cool down from 1210° C. to 20°C. at 600° C./hr. rate.

The appearance of the sintered wafers was similar to that of ComparativeExample A (“LAVA”). The wafers that were diced to a thickness of 1 mmwere polished on both faces. The samples were polished using polishingequipment comprised of an electrically driven head (“VECTOR POWER HEAD”)and a grinder-polisher (“BETA GRINDER-POLISHER”). The samples wereground flat on both sides using 30 micrometer diamond lapping film (“3MDIAMOND LAPPING FILM 668X”). Then 9 micrometer diamond lapping film (“3MDIAMOND LAPPING FILM 668X”) was used on both sides until the majority ofthe 30 micrometer scratches were removed. Next, the sample was polishedon both sides using 6 micrometers diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 9 micrometer scratches were removed. Finally, the samplewas polished on both sides using 3 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 6 micrometer scratches were removed.The polished samples were then characterized for their T/T_(L) (measuredusing the process described above for Comparative Example A),Archimedes' density, and phase composition, as determined by XRD, usingthe methods described above.

The sample was then set on a bed of zirconia beads in an aluminacrucible and thermally etched in air in a rapid temperature furnace (CMFurnaces Inc.), as follows: i—heat from 20° C. to 1160° C. at 450°C./hr. rate; ii—hold at 1160° C. for 1 hour; and iii—cool from 1160° C.to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described above.

The wafers that were diced to a thickness of 1.5 mm were polished on oneface in preparation for biaxial flexural strength testing according tothe test method above. The samples were polished using a 12 open facelapping machine “LAPMASTER”) for all but the final polishing step. Thesamples were all adhered to a sample plate and were then ground flatusing 20 micrometer diamond tile (“3M TRIZACT DIAMOND TILE”) at a speedof 30 rpm. The abrasive was then switched to 9 micrometer diamond tile(“3M TRIZACT DIAMOND TILE”) and grinding continued at 30 rpm until themajority of the 20 micrometer scratches were removed. The abrasive wasthen switched to 3 micrometer diamond tile (“3M TRIZACT DIAMOND TILE”)and grinding continued at 30 rpm until the majority of the 9 micrometerscratches were removed. The final polish was done using Buehlerpolishing equipment comprised of an electrically driven head (“VECTORPOWER HEAD”) and a grinder-polisher (“BETA GRINDER POLISHER”) and 3micrometer METADI diamond suspension (“DIAMOND SUSPENSION”) on apolishing cloth (“TEXMET POLISHING CLOTH”) until the majority of thescratches were removed. The properties of the sintered wafers are givenin Table 3, below.

TABLE 3 Pol- Archi- ished Phase medes Thick- Pol- Grain Compo- Exam-Density, ness, ished Size, Strength, sition ple g/cm³ mm T/T_(L) nm MPa(XRD) 3A 6.10 0.74 1.07 3B 6.10 0.69 1.06 127 [ZrO₂(T) a = 3.612, c =5.1 89 A° major] + [La₂Zr₂O₇ minor] 3B 0.97 1080 3C 0.97 1080 3D 0.971080

Example 4

A 76.2 gram sample of Sol B1 (prepared and diafiltered and concentratedas described above, 35.8 wt. % oxide and 3.2 wt. % acetic acid) wascharged in to a 500 ml RB flask. Water (26.5 grams) was removed viarotary evaporation resulting in a viscous somewhat dry material. Ethanol(15.3 grams), acrylic acid (2.88 grams) and HEMA (1.5 gram) and D.Iwater (0.4 gram) were added to the flask. The contents were stirredovernight resulting is a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 3 minutes). The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 4 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 4 was 17.9 grams. About 850 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 4 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 4 aerogel was semi-translucent with a bluishtint and weighed 9.6 grams, corresponding to an overall weight lossduring the supercritical extraction process of 46.4%.

Organic Burnout and Pre-Sinter Process

The extracted aerogel sample of Example 4 from above was removed fromits closed container and immediately set on a bed of zirconia beads inan unglazed porcelain crucible, covered with alumina fiberboard thenfired in air according to the following schedule in a high temperaturefurnace (“THERMOLYNE TYPE 46200”): i—heat from 20° C. to 225° C. at 18°C./hr. rate; ii—hold at 225° C. for 24 hours; iii—heat from 225° C. to400° C. at 6° C./hr. rate; iv—heat from 400° C. to 600° C. at 18° C./hr.rate; v—heat from 600° C. to 1090° C. at 120° C./hr. rate; and vi—cooldown from 1090° C. to 20° C. at 600° C./hr. rate.

After firing the sample was crack free. The cylinder was diced intoabout 2 mm thick wafers. The wafers were ion exchanged by first placingthem in a 118 ml glass jar containing distilled water at a depth ofabout 2.5 cm and then vacuum infiltrating. The water was replaced withabout a 2.5 cm depth of 1.0N NH₄OH and the wafers were soaked overnightfor 16 hours or longer. The NH₄OH was then poured off and the jar wasfilled with distilled water. The wafers were soaked in the distilledwater for 1 hour. The water was then replaced with fresh distilledwater. This step was repeated until the pH of the soak water was equalto that of fresh distilled water. The wafers were then dried at 90-125°C. for a minimum of 1 hour. The pre-sintered at 1090° C. aerogel ofExample 4 had 50.4 volume % of oxides. as determined by dividing thegeometric density of the pre-sintered wafer by the Archimedes density ofthe sintered wafer and then multiplying by 100.

Sintering Process

A wafer of Example 4 prepared as described above was set on a bed ofzirconia beads in an alumina crucible, covered with alumina fiberboardthen sintered in air according to the following schedule in a cruciblefurnace (Model 56724; “LINDBERG/BLUE M 1700° C.”): i—heat from 20° C. to1090° C. at 600° C./hr. rate; ii—heat from 1090° C. to 1250° C. at 120°C./hr. rate; iii—hold at 1250° C. for 2 hours; and iv—cool down from1250° C. to 20° C. at 600° C./hr. rate.

The wafer was polished on both faces using polishing equipment comprisedof an electrically driven head (“VECTOR POWER HEAD”) and agrinder-polisher (“BETA GRINDER-POLISHER). The sample was ground flat onboth sides using 30 micrometer diamond lapping film (“3M DIAMOND LAPPINGFILM 668X”). Then 9 micrometer diamond lapping film (“3M DIAMOND LAPPINGFILM 668X”) was used on both sides until the majority of the 30micrometer scratches were removed. Next the sample was polished on bothsides using 6 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”), until themajority of the 9 micrometer scratches were removed. Finally the samplewas polished on both sides using 3 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”), until the majority of the 6 micrometer scratches were removed.The polished sample was translucent and lines were distinct when thesample was placed directly on top of them and at a distance. The sampleappeared reddish in color in transmitted light and appeared bluish incolor in reflected light. The Archimedes density and T/T_(L) weremeasured as described in the test methods described above. The samplewas then set on a bed of zirconia beads in an alumina crucible andthermally etched in air in a Rapid Temperature Furnace as follows:i—heat from 20° C. to 1200° C. at 450° C./hr. rate; ii—hold at 1200° C.for 0.5 hour; and iii—cool from 1200° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described in the test method above.

The sintered Example 4 samples had an Archimedes density of 6.06 g/cm³,a polished T/T_(L) of 1.6 at a polished thickness of 1.2 mm, and anaverage grain size of 195 nm.

Example 5

A 29.3 gram sample of Sol C4 (prepared and diafiltered and concentratedas described above, 27.9 wt. % oxide and 3 wt. % acetic acid) and 196.5grams of Sol T2 (prepared and diafiltered and concentrated as describedabove, 23.6 wt. % oxide and 2.26 wt. % acetic acid) was charged in to a500 ml RB flask. Water (125.8 grams) was removed via rotary evaporationresulting in a viscous somewhat dry material. Ethanol (30.3 grams),acrylic acid (5.8 grams), HEMA (2.95 grams) were added to the flask. Thecontents were stirred overnight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.3 gram) was added andstirred until dissolved. The contents of the flask were then purged withN2 gas 6 minutes). The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Example 5 were removed separately from theethanol bath, weighed, placed individually inside small canvas pouches,and then stored briefly in another ethanol bath before being loaded intothe 10-L extractor vessel. The wet weight of Example 5A was 20.5 grams.The wet weight of Example 5B and Example 5C were 19.6 grams and 21.6grams, respectively. For extraction of all the gels of Example 5A-C,about 850-875 ml of 200-proof ethanol was added to the 10-L extractor ofa laboratory-scale supercritical fluid extractor unit. The canvas bagscontaining the wet zirconia-based gels were transferred from the ethanolbath into the 10-L extractor so that the wet gels were completelyimmersed in the liquid ethanol inside the jacketed extractor vessel,which was heated and maintained at 60° C. After the extractor vessel lidwas sealed in place, liquid carbon dioxide was pumped by a chilledpiston pump (setpoint: −12.5° C.) through a heat exchanger to heat theCO₂ to 60° C. and into the 10-L extractor vessel until an internalpressure of 11 MPa was reached. At these conditions, carbon dioxide issupercritical. Once the extractor operating conditions of 11 MPa and 60°C. were met, a PID-controlled needle valve regulated the pressure insidethe extractor vessel by opening and closing to allow the extractoreffluent to pass through a porous 316L stainless steel frit (obtainedfrom Mott Corporation as Model #1100S-5.480 DIA-.062-10-A), then througha heat exchanger to cool the effluent to 30° C., and finally into a 5-Lcyclone separator vessel that was maintained at room temperature andpressure less than 5.5 MPa, where the extracted ethanol and gas-phaseCO₂ were separated and collected throughout the extraction cycle forrecycling and reuse. Supercritical carbon dioxide (scCO₂) was pumpedcontinuously through the 10-L extractor vessel for 7 hours from the timethe operating conditions were achieved. After the 7-hour extractioncycle, the extractor vessel was slowly vented into the cyclone separatorover 16 hours from 11 MPa to atmospheric pressure at 60° C. before thelid was opened and the dried canvas pouches containing the aerogel wereremoved. The dry Example 5 aerogels were removed from their canvaspouches, weighed, and transferred into 237 ml glass jars packed withtissue paper for storage. The dry Example 5A aerogel wassemi-translucent with a bluish tint and weighed 10.8 grams,corresponding to an overall weight loss during the supercriticalextraction process of 47.3%. The dry Example 5B aerogel wassemi-translucent with a bluish tint and weighed 10.2 grams correspondingto an overall weight loss during the supercritical extraction process of48%. The dry Example 5C aerogel was semi-translucent with a bluish tintand weighed 11.3 grams corresponding to an overall weight loss duringthe supercritical extraction process of 47.7%.

Organic Burnout and Pre-Sinter Process

The extracted Example 5 aerogel samples from above were removed fromtheir closed container and the weight, diameter and height were measuredprior to being set on a bed of zirconia beads in an unglazed porcelaincrucible, covered with alumina fiberboard then fired in air according tothe following schedule in a high temperature furnace (“THERMOLYNE TYPE46200”): i—heat from 20° C. to 225° C. at 18° C./hr. rate; ii—hold at225° C. for 24 hour; iii—heat from 225° C. to 400° C. at 6° C./hr. rate;iv—heat from 400° C. to 600° C. at 18° C./hr. rate; v—heat from 600° C.to 1090° C. at 120° C./hr. rate; and vi—cool down from 1090° C. to 20°C. at 600° C./hr. rate.

After firing the samples were crack free. The samples of Example 5 werediced into about 2.5 mm thick wafers. The wafers were ion exchanged byfirst placing them in a 118 ml glass jar containing distilled water at adepth of about 2.5 cm and then vacuum infiltrating. The water wasreplaced with about a 2.5 cm depth of 1.0N NH₄OH and the wafers weresoaked overnight for 16 hours or longer. The NH₄OH was then poured offand the jar was filled with distilled water. The wafers were soaked inthe distilled water for 1 hour. The water was then replaced with freshdistilled water. This step was repeated until the pH of the soak waterwas equal to that of fresh distilled water. The wafers were then driedat 125° C. for a minimum of 1 hour. The aerogel of Example 5A had 11.8volume % of oxides while the pre-sintered (at 1090° C.) aerogel ofExample 5A had 50.4 volume % of oxides. The aerogel of Example 5B had 12volume % of oxides while the pre-sintered (at 1090° C.) aerogel ofExample 5B had 49.8 volume % of oxides. The aerogel of Example δC had11.9 volume % of oxides while the pre-sintered (at 1090° C.) aerogel ofExample 5C had 49.7 volume % of oxides. The volume percent oxide valueswere calculated using the method described above.

Sintering Process

The wafers prepared as described above were set on a bed of zirconiabeads in an alumina crucible, covered with alumina fiberboard thensintered in air according to the following schedule in a cruciblefurnace (Model 56724; (“LINDBERG/BLUE M 1700° C.”): i—heat from 20° C.to 1090° C. at 600° C./hr. rate; ii—heat from 1090° C. to 1250° C. at120° C./hr. rate; iii—hold at 1250° C. for 2 hours; and iv—cool downfrom 1250° C. to 20° C. at 600° C./hr. rate.

The 1 mm thick Example 5A sample wafer was polished on both faces usingBuehler polishing equipment comprised of an electrically driven head(“VECTOR POWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”).The sample was ground flat on both sides using 30 micrometer diamondlapping film (“3M DIAMOND LAPPING FILM 668X”). Then 9 micrometer diamondlapping film (“3M DIAMOND LAPPING FILM 668X”) was used on both sidesuntil the majority of the 30 micrometer scratches were removed. Next thesample was polished on both sides using 6 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 9 micrometer scratches were removed.Finally, the sample was polished on both sides using 3 micrometerdiamond suspension (“METADI DIAMOND SUSPENSION”) on a polishing cloth(“TEXMET POLISHING CLOTH”) until the majority of the 6 micrometerscratches were removed. The polished sample was translucent and lineswere distinct when the sample was placed directly on top of them and ata distance. The sample appeared reddish in color in transmitted lightand appeared bluish in color in reflected light. The Archimedes densityand T/T_(L) were measured as determined by the method described above.The sintered Example 5A sample had an Archimedes density of 6.07 g/cm³,and a polished T/T_(L) of 1.1 at a polished thickness of 0.63 mm.

The Example 5B and Example 5C samples 2.5 mm wafers were polished on oneface using a 12 open face lapping machine (“LAPMASTER”) for all but thefinal polishing step. The biaxial flexural strength was measured on the2.5 mm samples after polishing using the test method above. The sampleswere all adhered to a sample plate and were then ground flat using 20micrometer diamond tile (“3M TRIZACT DIAMOND TILE”) at a speed of 30rpm. The abrasive was then switched to 9 micrometer diamond tile (“3MTRIZACT DIAMOND TILE”) and grinding continued at 30 rpm until themajority of the 20 micrometer scratches were removed. The abrasive wasthen switched to 3 micrometer diamond tile (“3M TRIZACT DIAMOND TILE”)and grinding continued at 30 rpm until the majority of the 9 micrometerscratches were removed. The final polish was done using polishingequipment comprised of an electrically driven head (“VECTOR POWER HEAD”)and a grinder-polisher (“BETA GRINDER-POLISHER”) and 3 micrometerdiamond suspension (“METADI DIAMOND SUSPENSION”) on a polishing cloth(“TEXMET POLISHING CLOTH”) until the majority of the scratches wereremoved. The average biaxial flexural strength was measured to be 1305MPa using the test method described above.

Example 6

A 39 gram sample of Sol C4 (prepared and diafiltered and concentrated asdescribed above, 27.9 wt. % oxide and 3 wt. % acetic acid) and 184.9grams of Sol T2 (prepared and diafiltered and concentrated as describedabove, 23.6 wt. % oxide and 2.3 wt. % acetic acid) was charged in to a500 ml RB flask. Water (123.9 grams) was removed via rotary evaporationresulting in viscous somewhat dry material. Ethanol (30.3 grams),acrylic acid (5.8 grams), HEMA (3 grams) were added to the flask. Thecontents were stirred overnight resulting is a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.3 gram) was added andstirred until dissolved. The contents of the flask were then purged withN2 gas for 6 minutes). The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Example 6 were removed separately from theethanol bath, weighed, placed individually inside small canvas pouches,and then stored briefly in another ethanol bath before being loaded intothe 10-L extractor vessel. The wet weight of sample Example 6A was 19.5grams. The wet weight of sample Example 6B was 19.3 grams. The wetweight of sample Example 6C was 19.5 grams. For extraction of all thegels of Example 6, about 850-875 ml of 200-proof ethanol was added tothe 10-L extractor of a laboratory-scale supercritical fluid extractor.The canvas bags containing the wet zirconia-based gels were transferredfrom the ethanol bath into the 10-L extractor so that the wet gels werecompletely immersed in the liquid ethanol inside the jacketed extractorvessel, which was heated and maintained at 60° C. The Example 6 sampleswere subjected to the same extraction process as described above for theExample 5 samples. Afterwards, the dry aerogels were removed from theircanvas pouches, weighed, and transferred into 237 ml glass jar packedwith tissue paper for storage. The dry Example 6A aerogel wassemi-translucent with a bluish tint and weighed 10.4 grams,corresponding to an overall weight loss during the supercriticalextraction process of 46.7%. The dry Example 6B aerogel wassemi-translucent with a bluish tint and weighed 10.2 grams correspondingto an overall weight loss during the supercritical extraction process of47.2%. The dry Example 6C aerogel was semi-translucent with a bluishtint and weighed 10.3 grams corresponding to an overall weight lossduring the supercritical extraction process of 47.2%.

Organic Burnout and Pre-Sinter Process

The extracted Example 6 aerogel samples from above were removed fromtheir closed container and the weight, diameter and height were measuredprior to being set on a bed of zirconia beads in an unglazed porcelaincrucible, covered with alumina fiberboard then fired in air according tothe following schedule in a high temperature furnace (“THERMOLYNE TYPE46200”): i—heat from 20° C. to 225° C. at 18° C./hr. rate; ii—hold at225° C. for 24 hours; iii—heat from 225° C. to 400° C. at 6° C./hr.rate; iv—heat from 400° C. to 600° C. at 18° C./hr. rate; v—heat from600° C. to 1090° C. at 120° C./hr. rate; and vi—cool down from 1090° C.to 20° C. at 600° C./hr. rate.

After firing, the samples were crack free. The samples of Example 6 werediced into about 1 mm or 2.5 mm thick wafers. The wafers were ionexchanged by first placing them in a 118 ml glass jar containingdistilled water at a depth of about 2.5 cm and then vacuum infiltrating.The water was replaced with about a 2.5 cm depth of 1.0N NH₄OH and thewafers were soaked overnight for 16 hours or longer. The NH₄OH was thenpoured off and the jar was filled with distilled water. The wafers weresoaked in the distilled water for 1 hour. The water was then replacedwith fresh distilled water. This step was repeated until the pH of thesoak water was equal to that of fresh distilled water. The wafers werethen dried (90° C. to 125° C.) for a minimum of 1 hour. The aerogel ofExample 6A had 12.2 volume % of oxides while the pre-sintered at 1090°C. aerogel of Example 6A had 51.4 volume % of oxides. The aerogel ofExample 6B had 12.4 volume % of oxides while the pre-sintered (at 1090°C.) aerogel of Example 6B had 50.4 volume % of oxides. The aerogel ofExample 6C had 12.35 volume % of oxides while the pre-sintered (at 1090°C.) aerogel of Example 6C had 49.8 volume % of oxides. The volumepercent oxide values were calculated using the method described above.

Sintering Process

The wafers prepared above were set on a bed of zirconia beads in analumina crucible, covered with alumina fiberboard then sintered in airaccording to the following schedule in a crucible furnace (Model 56724;“LINDBERG/BLUE M 1700° C.”): i—heat from 20° C. to 1090° C. at 600°C./hr. rate; ii—heat from 1090° C. to 1250° C. at 120° C./hr. rate;iii—hold at 1250° C. for 2 hours; and iv—cool down from 1250° C. to 20°C. at 600° C./hr. rate.

Then the 1 mm thick Example 6A wafer was polished on both faces usingBuehler polishing equipment comprised of an electrically driven head(“VECTOR POWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”).The sample was ground flat on both sides using 30 micrometer diamondlapping film (“3M DIAMOND LAPPING FILM 668X”). Then 9 micrometer diamondlapping film (“3M DIAMOND LAPPING FILM 668X”) was used on both sidesuntil the majority of the 30 micrometer scratches were removed. Next thesample was polished on both sides using 6 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 9 micrometer scratches were removed.Finally the sample was polished on both sides using 3 micrometer diamondsuspension (“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMETPOLISHING CLOTH”) until the majority of the 6 micrometer scratches wereremoved. The polished sample was translucent and lines were distinctwhen the sample was placed directly on top of them and at a distance.The sample appeared reddish in color in transmitted light and appearedbluish in color in reflected light. The Archimedes density and T/T_(L)were measured as described above. The sintered Example 6A sample had anArchimedes density of 6.05 g/cm³, and a polished T/T_(L) of 1.15 at apolished thickness of 0.65 mm.

The Example 6B and Example 6C samples 2.5 mm wafers were polished on oneface using a 12 open face lapping machine (“LAPMASTER”) for all but thefinal polishing step. The biaxial flexural strength was measured on the2.5 mm samples after polishing using the test method above. The sampleswere all adhered to a sample plate and were then ground flat using 20micrometer diamond tile (“3M TRIZACT DIAMOND TILE”) at a speed of 30rpm. The abrasive was then switched to 9 micrometer diamond tile (“3MTRIZACT DIAMOND TILE”) and grinding continued at 30 rpm until themajority of the 20 micrometer scratches were removed. The abrasive wasthen switched to 3 micrometer diamond tile (“3M TRIZACT DIAMOND TILE”)and grinding continued at 30 rpm until the majority of the 9 micrometerscratches were removed. The final polish was done using Buehlerpolishing equipment comprised of an electrically driven head (“VECTORPOWER HEAD”) and a grinder-polisher (“BETA GRINDER POLISHER”) and 3micrometer METADI diamond suspension (“DIAMOND SUSPENSION”) on apolishing cloth (“TEXMET POLISHING CLOTH”) until the majority of thescratches were removed. The average biaxial flexural strength wasmeasured to be 1202 MPa using the test method described above.

Example 7 and 8

For Example 7, a 23.3 gram sample of Sol C3 (prepared and diafilteredand concentrated as described above, 29.5 wt. % oxide and 3.1 wt. %acetic acid) and 32.4 grams of Sol T2 (prepared and diafiltered andconcentrated as described above, 54.7 wt. % oxide and about 5.5 wt. %acetic acid) was charged in to a 500 ml RB flask. Water (7.9 grams) wasremoved via rotary evaporation resulting in a viscous somewhat drymaterial. Ethanol (18.2 grams), acrylic acid (2.9 grams) and HEMA (1.46gram) were added to the flask. The contents were stirred overnightresulting in a fluid translucent sol. 2,2′-azobis(2-methylbutyronitrile)(“VAZO 67”) (0.15 gram) was added and stirred until dissolved. Thecontents of the flask were then purged with N₂ gas for 3 minutes. Thesample (translucent and low viscosity) was charged to cylindricalcontainers (29 mm diameter). Each container was about 18 ml in volumeand each was sealed on both ends (very little air gap was left betweenthe top and liquid). The samples were allowed to stand for about 1 hourthen placed in an oven to cure (50° C., 4 hours). This results in aclear translucent blue gel. The gel was removed from the container andplaced in a 473 ml wide mouth jar. The jar was filled with ethanol(denatured). The sample was soaked for 24 hours then the ethanol wasreplaced with fresh ethanol. The sample was soaked for 24 hours then theethanol was replaced with a third batch of fresh ethanol. The sample wasallowed to soak until the supercritical extraction was done. The abovemanipulations were done minimizing the amount of time the gel wasexposed to the air.

For Example 8, a 48.78 gram sample of Sol C4 (prepared and diafilteredand concentrated as described above, 27.9 wt. % oxide and 3 wt. % aceticacid) and 153.2 grams of Sol T2 (prepared and diafiltered andconcentrated as described above, 26.6 wt. % oxide and 2.55 wt. % aceticacid) was charged in to a 500 ml RB flask. Water (102.7 grams) wasremoved via rotary evaporation resulting in a viscous somewhat drymaterial. Ethanol (30.3 grams), acrylic acid (5.8 grams) HEMA (2.9grams) and DI water (0.7 gram) were added to the flask. The contentswere stirred overnight resulting is a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.3 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 6 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Examples 7 and 8 were removed separately fromthe ethanol bath, weighed, placed individually inside small canvaspouches, and then stored briefly in another ethanol bath before beingloaded into the 10-L extractor vessel. The wet weight of sample Example7A was 21.5 grams. The wet weight of sample Example 7B was 20.5 grams.The wet weight of sample Example 7C was 19.9 gram. The wet weight ofsample Example 8 was 20.2 grams. For extraction of all the gels ofExamples 7 and 8, about 800 ml of 200-proof ethanol was added to the10-L extractor of a laboratory-scale supercritical fluid extractor. Thecanvas bags containing the wet zirconia-based gels were transferred fromthe ethanol bath into the 10-L extractor so that the wet gels werecompletely immersed in the liquid ethanol inside the jacketed extractorvessel, which was heated and maintained at 60° C. The Example 7A, 7B,7C, and Example 8 samples were subjected to the same extraction processas described above for Examples 1 and 2 samples. Afterwards, the dryaerogels were removed from their canvas pouches, weighed, andtransferred into individual 237 ml glass jars packed with tissue paperfor storage. The dry Example 7A aerogel was semi-translucent with abluish tint and weighed 11.5 grams, corresponding to an overall weightloss during the supercritical extraction process of 46.5%. The dryExample 7B aerogel was semi-translucent with a bluish tint and weighed11.1 grams, corresponding to an overall weight loss during thesupercritical extraction process of 45.9%. The dry Example 7C aerogelwas semi-translucent with a bluish tint and weighed 10.7 grams,corresponding to an overall weight loss during the supercriticalextraction process of 46.2%. The dry Example 8 aerogel wassemi-translucent with a bluish tint and weighed 11.1 grams,corresponding to an overall weight loss during the supercriticalextraction process of 45%.

Organic Burnout and Pre-Sinter Process

The extracted Example 7A and 7B aerogel samples prepared above wereremoved from their closed containers and dried for 1 hour in open airprior to being set on a bed of zirconia beads in an unglazed porcelaincrucible, covered with alumina fiberboard then fired in air according tothe following schedule in a high temperature furnace (“THERMOLYNE TYPE46200”): i—heat from 20° C. to 225° C. at 18° C./hr. rate; ii—hold at225° C. for 24 hours; iii—heat from 225° C. to 400° C. at 6° C./hr.rate; iv—heat from 400° C. to 600° C. at 18° C./hr. rate; v—heat from600° C. to 1090° C. at 120° C./hr. rate; and vi—cool down from 1090° C.to 20° C. at 600° C./hr. rate.

After firing the samples were crack free. The cylinders were diced intoabout 1 mm or 2.5 mm thick wafers. The samples of Examples 7A and 7Bwafers were ion exchanged by first placing them in a 118 ml glass jarcontaining distilled water at a depth of about 2.5 cm and then vacuuminfiltrating. The water was replaced with about a 2.5 cm depth of 1.0NNH₄OH and the wafers were soaked overnight for 16 hours or longer. TheNH₄OH was then poured off and the jar was filled with distilled water.The wafers were soaked in the distilled water for 1 hour. The water wasthen replaced with fresh distilled water. This step was repeated untilthe pH of the soak water was equal to that of fresh distilled water. Thewafers were then dried at 90-125° C. for a minimum of 1 hour.

The extracted aerogel sample of Example 7C prepared above was analyzedto determine the BET surface area, pore size and porosity. The aerogelof Example 7C had a 222 m²/g of surface area MBET, 0.826 cm³/g of totalpore volume and 149 Angstrom of average pore diameter.

Example 8 samples had the same organic burnout and pre-sinter conditionsas Examples 7A and 7B except it was not dried in open air prior toorganic burnout and pre-sinter. Also, half of the Example 8 wafers wereion exchanged as described for Examples 7A and 7B and the other halfwere not.

The pre-sintered at 1090° C. aerogels of Examples 7A, 7B and Example 8A(ion-exchanged) and Example 8B (not ion-exchanged) had 45.2, 47, 44.6,and 44.5 volume % of oxides, respectively as determined by dividing thegeometric density of the pre-sintered wafer by the Archimedes density ofthe sintered wafer and then multiplying by 100.

Sintering Process

Wafers were set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heatfrom 1090° C. to 1250° C. at 120° C./hr. rate; iii—hold at 1250° C. for2 hours; and iv—cool down from 1250° C. to 20° C. at 600° C./hr. rate.

An as fired 7A wafer was submitted for XRD. The 1 mm wafers werepolished on both faces and the 2.5 mm wafers were polished on one faceusing a 12 open face lapping machine (“LAPMASTER”) for all but the finalpolishing step. The biaxial flexural strength was measured on the 2.5 mmsamples after polishing using the test method above. The samples wereall adhered to a sample plate and were then ground flat using a 20micrometer diamond tile (“3M TRIZACT DIAMOND TILE”) at a speed of 30rpm. The abrasive was then switched to a 9 micrometer diamond tile (“3MTRIZACT DIAMOND TILE”) and grinding continued at 30 rpm until themajority of the 20 micrometer scratches were removed. The abrasive wasthen switched to a 3 micrometer diamond tile (“3M TRIZACT DIAMOND TILE”)and grinding continued at 30 rpm until the majority of the 9 micrometerscratches were removed. The final polish was done using polishingequipment comprised of an electrically driven head (“VECTOR POWER HEAD”)and a grinder-polisher (“BETA GRINDER-POLISHER”) and 3 micrometerdiamond suspension (“METADI DIAMOND SUSPENSION”) on a polishing cloth(“TEXMET POLISHING CLOTH”) until the majority of the scratches wereremoved. The samples polished on both faces were translucent and lineswere distinct when the samples were placed directly on top of them andat a distance. The samples appeared reddish in color in transmittedlight and appeared bluish in color in reflected light. The Example 8Bsample that was not ion exchanged exhibited a slight tan color that wasnot seen in the sample that was ion exchanged. The Archimedes densityand T/T_(L) were measured as described above. After strength testing apiece of sintered Example 7B was set on a bed of zirconia beads in analumina crucible and thermally etched in air in a rapid temperaturefurnace (CM Furnaces Inc.) as follows: i—heat from 20° C. to 1200° C. at450° C./hr. rate; ii—hold at 1200° C. for 0.5 hour; and iii—cool from1200° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described in the test method above.

The properties of the sintered wafers of Examples 7A, 7B, Example 8A(ion-exchanged) and Example 8B (not ion-exchanged) are given in Table 4,below.

TABLE 4 Pol- Archi- ished Phase medes Thick- Pol- Grain Compo- Exam-Density, ness, ished Size, Strength, sition ple g/cm³ mm T/T_(L) nm MPa(XRD) 7A 6.03 0.31 1.46 [ZrO2(T) major a = 3.628 c = 5.179] 7B 6.07 1.71156 1323 8A 6.05 0.55 1.36 8B 6.05 0.49 1.35

Example 9

For Example 9, a 68.25 gram sample of Sol C4 (prepared, diafiltered andconcentrated as described above, 27.9 wt. % oxide and 3 wt. % aceticacid) and 150.4 gram of Sol T2 (prepared and diafiltered andconcentrated as described above, 23.55 wt. % oxide and 2.3 wt. % aceticacid) was charged in to a 500 ml RB flask. Water (118.6 grams) wasremoved via rotary evaporation resulting in a viscous somewhat drymaterial. Ethanol (30.3 grams), acrylic acid (5.8 grams) HEMA (2.9grams) and DI water (0.7 gram) were added to the flask. The contentswere stirred overnight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.3 gram) was added andstirred until dissolved. The contents of the flask were then purged withN2 gas for 6 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 nil in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂ based gels of Example 9 were removed separately from theethanol bath, weighed, placed individually inside small canvas pouches,and then stored briefly in another ethanol bath before being loaded intothe 10-L extractor vessel. The wet weight of Example 9A was 20.4 grams.The wet weight of Example 9B was 21.3 grams. The wet weight of Example9C was 21.1 grams. For extraction on all the gels of Example 9, about850-875 ml of 200-proof ethanol was added to the 10-L extractor of alaboratory-scale supercritical fluid extractor unit. The canvas bagscontaining the wet zirconia-based gels were transferred from the ethanolbath into the 10-L extractor so that the wet gels were completelyimmersed in the liquid ethanol inside the jacketed extractor vessel,which was heated and maintained at 60° C. The Example 9 samples weresubjected to the same extraction process as described above for theExample 5 sample. Afterwards, the dry aerogels were removed from theircanvas pouches, weighed, and transferred into a 237 ml glass jar packedwith tissue paper for storage. The dry Example 9A aerogel wassemi-translucent with a bluish tint and weighed 10.9 grams,corresponding to an overall weight loss during the supercriticalextraction process of 46.6%. The dry Example 9B aerogel wassemi-translucent with a bluish tint and weighed 11.3 grams correspondingto an overall weight loss during the supercritical extraction process of47%. The dry Example 9C aerogel was semi-translucent with a bluish tintand weighed 11.2 grams corresponding to an overall weight loss duringthe supercritical extraction process of 46.9%.

Organic Burnout and Pre-Sinter Process

The extracted Example 9 aerogel samples from above were removed fromtheir closed containers and the weight, diameter and height weremeasured prior to being set on a bed of zirconia beads in an unglazedporcelain crucible, covered with alumina fiberboard then fired in airaccording to the following schedule in a high temperature furnace(“THERMOLYNE TYPE 46200”): i—heat from 20° C. to 225° C. at 18° C./hr.rate; ii—hold at 225° C. for 24 hours; iii—heat from 225° C. to 400° C.at 6° C./hr. rate; iv—heat from 400° C. to 600° C. at 18° C./hr. rate;v—heat from 600° C. to 1090° C. at 120° C./hr. rate; and vi—cool downfrom 1090° C. to 20° C. at 600° C./hr. rate.

After Firing the samples were crack free. The samples of Example 9 werediced into about 1 mm or 2.5 mm thick wafers. The wafers were ionexchanged by first placing them in a 118 ml glass jar containingdistilled water at a depth of about 2.5 cm and then vacuum infiltrating.The water was replaced with about a 2.5 cm depth of 1.0N NH₄OH and thewafers were soaked overnight for 16 hours or longer. The NH₄OH was thenpoured off and the jar was filled with distilled water. The wafers weresoaked in the distilled water for 1 hour. The water was then replacedwith fresh distilled water. This step was repeated until the pH of thesoak water was equal to that of fresh distilled water. The wafers werethen dried (at 90° C. to 125° C.) for a minimum of 1 hour. The aerogelof Example 9A had 12 volume % of oxides while the pre-sintered (at 1090°C.) aerogel of Example 9A had 49.3 volume % of oxides. The aerogel ofExample 9B had 12.1 volume % of oxides while the pre-sintered (at 1090°C.) aerogel of Example 9B had 47.9 volume % of oxides. The aerogel ofExample 9C had 12 volume % of oxides while the pre-sintered (at 1090°C.) aerogel of Example 9C had 47.8 volume % of oxides. The volumepercent oxide values were calculated using the method described above.

Sintering Process

The wafers prepared as described above were set on a bed of zirconiabeads in an alumina crucible, covered with alumina fiberboard thensintered in air according to the following schedule in a cruciblefurnace (Model 56724; “LINDBERG/BLUE M 1700° C.”): i—heat from 20° C. to1090° C. at 600° C./hr. rate; ii—heat from 1090° C. to 1250° C. at 120°C./hr. rate; iii—hold at 1250° C. for 2 hours; and iv—cool down from1250° C. to 20° C. at 600° C./hr. rate.

The 1 mm thick Example 9A wafer was polished on both faces usingpolishing equipment comprised of an electrically driven head (“VECTORPOWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”). Thesample was ground flat on both sides using 30 micrometer diamond lappingfilm (“3M DIAMOND LAPPING FILM 668X”). Then 9 micrometer diamond lappingfilm (“3M DIAMOND LAPPING FILM 668X”) was used on both sides until themajority of the 30 micrometer scratches were removed. Next the samplewas polished on both sides using 6 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 9 micrometer scratches were removed.Finally the sample was polished on both sides using 3 micrometer diamondsuspension (“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMETPOLISHING CLOTH”) until the majority of the 6 micrometer scratches wereremoved. The polished sample was translucent and lines were distinctwhen the sample was placed directly on top of them and at a distance.The sample appeared reddish in color in transmitted light and appearedbluish in color in reflected light. The Archimedes density and T/T_(L)were measured as described above. The sintered Example 9A samples had anArchimedes density of 6.03 g/cm³, a polished T/T_(L) of 1.5 at apolished thickness of 0.66 mm.

The Example 9B and Example 9C samples 2.5 mm wafers were polished on oneface using a 12 open face lapping machine “LAPMASTER”) for all but thefinal polishing step. The biaxial flexural strength was measured on the2.5 mm samples after polishing using the test method above. The sampleswere all adhered to a sample plate and were then ground flat using 20micrometer diamond tile (“3M TRIZACT DIAMOND TILE”) at a speed of 30rpm. The abrasive was then switched to 9 micrometer diamond tile (“3MTRIZACT DIAMOND TILE”) and grinding continued at 30 rpm until themajority of the 20 micrometer scratches were removed. The abrasive wasthen switched to 3 micrometer diamond tile (“3M TRIZACT DIAMOND TILE”)and grinding continued at 30 rpm until the majority of the 9 micrometerscratches were removed. The final polish was done using Buehlerpolishing equipment comprised of an electrically driven head (“VECTORPOWER HEAD”) and a grinder-polisher (“BETA GRINDER POLISHER”) and 3micrometer METADI diamond suspension (“DIAMOND SUSPENSION”) on apolishing cloth (“TEXMET POLISHING CLOTH”) until the majority of thescratches were removed. The average biaxial flexural strength wasmeasured to be 757 MPa using the test method described above.

Example 10

For Examples 10A and 10B, a 46.3 gram sample of Sol C3 (prepared anddiafiltered and concentrated as described above, 29.5 wt. % oxide and3.1 wt. % acetic acid) and 24.9 grams of Sol T2 (prepared anddiafiltered and concentrated as described above, 54.7 wt. % oxide andabout 5.5 wt. % acetic acid) was charged in to a 500 ml RB flask. Water(26.8 grams) was removed via rotary evaporation resulting in a viscoussomewhat dry material. Ethanol (20.7 grams), acrylic acid (2.9 grams)HEMA (1.47 gram) were added to the flask. The contents were stirredovernight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 3 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hr then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Example 10A and 10B were removed separatelyfrom the ethanol bath, weighed, placed individually inside small canvaspouches, and then stored briefly in another ethanol bath before beingloaded into the 10-L extractor vessel. The wet weight of Example 10A was19.4 grams. The wet weight of sample Example 10B was 21.6 grams. For theextraction of both Examples 10A and 10B, about 800 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bags containing the wetzirconia-based gels were transferred from the ethanol bath into the 10-Lextractor so that the wet gels were completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 10A and 10B samples were subjected tothe same extraction process as described above for Examples 1 and 2samples. Afterwards, the dry aerogels were removed from their canvaspouches, weighed, and transferred into individual 237 ml glass jarspacked with tissue paper for storage. The dry Example 10A aerogel wassemi-translucent with a bluish tint and weighed 10.3 grams,corresponding to an overall weight loss during the supercriticalextraction process of 46.9%. The dry Example 10B aerogel wassemi-translucent with a bluish tint and weighed 11.5 grams,corresponding to an overall weight loss during the supercriticalextraction process of 46.8%.

Organic Burnout and Pre-Sinter Process

The extracted Example 10A and 10B aerogel samples from above wereremoved from their closed containers and dried for 1 hour in open airprior to being set on a bed of zirconia beads in an unglazed porcelaincrucible, covered with alumina fiberboard then fired in air according tothe following schedule in a high temperature furnace (“THERMOLYNE TYPE46200”): i—heat from 20° C. to 225° C. at 18° C./hr. rate; ii—hold at225° C. for 24 hours; iii—heat from 225° C. to 400° C. at 6° C./hr.rate; iv—heat from 400° C. to 600° C. at 18° C./hr. rate; v—heat from600° C. to 1090° C. at 120° C./hr. rate; and vi—cool down from 1090° C.to 20° C. at 600° C./hr. rate.

After firing the samples were crack free. The cylinders were diced intoabout 1 mm or 2.5 mm thick wafers. The samples of Example 10A and 10Bwafers were ion exchanged by first placing them in a 118 ml glass jarcontaining distilled water at a depth of about 2.5 cm and then vacuuminfiltrating. The water was replaced with about a 2.5 cm depth of 1.0NNH₄OH and the wafers were soaked overnight for 16 hours or longer. TheNH₄OH was then poured off and the jar was filled with distilled water.The wafers were soaked in the distilled water for 1 hour. The water wasthen replaced with fresh distilled water. This step was repeated untilthe pH of the soak water was equal to that of fresh distilled water. Thewafers were then dried at 90-125° C. for a minimum of 1 hour. Thepre-sintered at 1090° C. aerogels of Example 10A and 10B had 45.6 and46.2 volume % of oxides, respectively. as determined by dividing thegeometric density of the pre-sintered wafer by the Archimedes density ofthe sintered wafer and then multiplying by 100.

Sintering Process

Wafers were set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heatfrom 1090° C. to 1250° C. at 120° C./hr. rate; iii—hold at 1250° C. for2 hours; and iv—cool down from 1250° C. to 20° C. at 600° C./hr. rate.

A sample of Example 10A as-fired wafer was analyzed using XRD. The 1 mmwafers were polished on both faces and the 2.5 mm wafers were polishedon one face using a 12 open face lapping machine (“LAPMASTER”) for allbut the final polishing step. The biaxial flexural strength was measuredon the 2.5 mm samples after polishing using the test method above. Thesamples were all adhered to a sample plate and were then ground flatusing 20 micrometer diamond tile (“3M TRIZACT DIAMOND TILE”) at a speedof 30 rpm. The abrasive was then switched to 9 micrometer diamond tile(“3M TRIZACT DIAMOND TILE”) and grinding continued at 30 rpm until themajority of the 20 micrometer scratches were removed. The abrasive wasthen switched to 3 micrometer diamond tile (“3M TRIZACT DIAMOND TILE”)and grinding continued at 30 rpm until the majority of the 9 micrometerscratches were removed. The final polish was done using Buehlerpolishing equipment comprised of an electrically driven head (“VECTORPOWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”) and 3micrometer diamond suspension (“METADI DIAMOND SUSPENSION”) on apolishing cloth (“TEXMET POLISHING CLOTH”) until the majority of thescratches were removed. The samples polished on both faces weretranslucent and lines were distinct when the samples were placeddirectly on top of them and at a distance. The samples appeared reddishin color in transmitted light and appeared bluish in color in reflectedlight. The Archimedes density and T/T_(L) were measured as describedabove. After strength testing a piece of aerogel of Example 10B was seton a bed of zirconia beads in an alumina crucible and thermally etchedin air in a rapid temperature furnace (CM Furnaces Inc.) as follows:i—heat from 20° C. to 1200° C. at 450° C./hr. rate; ii—hold at 1200° C.for 0.5 hour; and iii—cool from 1200° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described in the test methods above.

The properties of the sintered wafers are given in Table 5, below.

TABLE 5 Pol- Archi- ished medes Thick- Pol- Grain Compo- Exam- Density,ness, ished Size, Strength, sition ple g/cm³ mm T/T_(L) nm MPa (XRD) 10A6.02 0.52 1.77 [ZrO2(T) major a = 3.637 c = 5.177] 10B 6.05 1.71 183 494

Example 11

For Examples 11A and 11B, a 30.2 gram sample of Sol C4 (prepared anddiafiltered and concentrated as described above, 27.9 wt. % oxide and3.05 wt. % acetic acid) and 52.55 grams of Sol B1 (prepared anddiafiltered and concentrated as described above, 35.8 wt. % oxide and3.2 wt. % acetic acid) was charged in to a 500 ml RB flask. Water (39.3grams) was removed via rotary evaporation resulting in a viscoussomewhat dry material. Ethanol (15.15 grams), acrylic acid (1.5 gram),HEMA (1.5 gram) and DI water (1.2 gram) were added to the flask. Thecontents were stirred overnight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 3 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This resulted in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Examples 11A and 11B were removed separatelyfrom the ethanol bath, weighed, placed individually inside small canvaspouches, and then stored briefly in another ethanol bath before beingloaded into the 10-L extractor vessel. The wet weight of Example 11A was18.7 gram. The wet weight of Example 11B was 19.9 grams. About 850 ml of200-proof ethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bags containing the wetzirconia-based gels were transferred from the ethanol bath into the 10-Lextractor so that the wet gels were completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 11A and 11B samples were subjected tothe same extraction process as described above for Examples 1 and 2samples. Afterwards, the dry aerogels were removed from their cam/aspouches, weighed, and transferred into individual 237 ml glass jarspacked with tissue paper for storage. The dry Example 11A aerogel wassemi-translucent with a bluish tint and weighed 10.4 grams,corresponding to an overall weight loss during the supercriticalextraction process of 44.4%. The dry Example 11B aerogel wassemi-translucent with a bluish tint and weighed 11.2 grams,corresponding to an overall weight loss during the supercriticalextraction process of 43.7%.

Organic Burnout and Pre-Sinter Process

The extracted Example 11A and 11B aerogel samples from above wereremoved from their closed containers and immediately set on a bed ofzirconia beads in an unglazed porcelain crucible. covered with aluminafiberboard then fired in air according to the following schedule in ahigh temperature furnace (“THERMOLYNE TYPE 46200”): i—heat from 20° C.to 225° C. at 18° C./hr. rate; ii—hold at 225° C. for 24 hours; iii—heatfrom 225° C. to 400° C. at 6° C./hr. rate; iv—heat from 400° C. to 600°C. at 18° C./hr. rate; v—heat from 600° C. to 1090° C. at 120° C./hr.rate; and vi—cool down from 1090° C. to 20° C. at 600° C./hr. rate.

After firing, the samples were crack free. The Example 11A cylinder wasdiced into about 1 mm or 2 mm thick wafers. The Example 11A wafers wereion exchanged by first placing them in a 118 ml glass jar containingdistilled water at a depth of about 2.5 cm and then vacuum infiltrating.The water was replaced with about a 2.5 cm depth of 1.0N NH₄OH and thewafers were soaked overnight for 16 hours or longer. The NH₄OH was thenpoured off and the jar was filled with distilled water. The wafers weresoaked in the distilled water for 1 hour. The water was then replacedwith fresh distilled water. This step was repeated until the pH of thesoak water was equal to that of fresh distilled water. The wafers werethen dried at 90-125° C. for a minimum of 1 hour. The pre-sintered at1090° C. aerogel of Example 11A had 47.8 volume % of oxides, asdetermined by dividing the geometric density of the pre-sintered waferby the Archimedes density of the sintered wafer and then multiplying by100.

Sintering Process

2 mm wafers of Example 11A were set on a bed of zirconia beads in analumina crucible, covered with alumina fiberboard then sintered in airaccording to the following schedule in a crucible furnace (Model 56724;“LINDBERG/BLUE M 1700° C.”): i—heat from 20° C. to 1090° C. at 600°C./hr. rate; ii—heat from 1090° C. to 1250° C. at 120° C./hr. rate;iii—hold at 1250° C. for 2 hours; and iv—cool down from 1250° C. to 20°C. at 600° C./hr. rate.

A wafer was polished on both faces using Buehler polishing equipmentcomprised of an electrically driven head (“VECTOR POWER HEAD”) and agrinder-polisher (“BETA GRINDER-POLISHER”). The sample was ground flaton both sides using 30 micrometer diamond lapping film (“3M DIAMONDLAPPING FILM 668X”). Then 9 micrometer diamond lapping film (“3M DIAMONDLAPPING FILM 668X”) was used on both sides until the majority of the 30micrometer scratches were removed. Next the sample was polished on bothsides using 6 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 9 micrometer scratches were removed. Finally the samplewas polished on both sides using 3 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 6 micrometer scratches were removed.The sample was translucent and lines were distinct when the sample wasplaced directly on top of them and at a distance. The sample appearedreddish in color in transmitted light and appeared bluish in color inreflected light. The Archimedes density and T/T_(L) were measured asdescribed above. The sample was set on a bed of zirconia beads in analumina crucible and thermally etched in air in a rapid temperaturefurnace (CM Furnaces Inc.) as follows: i—heat from 20° C. to 1200° C. at450° C./hr. rate; ii—hold at 1200° C. for 0.5 hour; and iii—cool from1200° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described in the test method above.

The sintered Example 11A sample had an Archimedes density of 6.02 g/cm³,a polished T/T_(L) of 2 at a polished thickness of 1.1 mm, and anaverage grain size of 202 nm.

Example 12 and 13

For Examples 12A and 12B, a 69.40 gram sample of Sol C3 (prepared anddiafiltered and concentrated as described above, 29.5 wt. % oxide and3.1 wt. % acetic acid) and 12.4 grams of Sol T2 (prepared anddiafiltered and concentrated as described above, 54.7 wt. % oxide andabout 5.5 wt. % acetic acid) was charged in to a 500 ml RB flask. Water(32.7 grams) was removed via rotary evaporation resulting in a viscoussomewhat dry material. Ethanol (16.1 grams), acrylic acid (2.9 grams),HEMA (1.47 gram) were added to the flask. The contents were stirredovernight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 3 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This results in clear translucent blue gels. The gels were removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The samples were soaked for 24 hr then theethanol was replaced with fresh ethanol. The samples were soaked for 24hr then the ethanol was replaced with a third batch of fresh ethanol.The samples were allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gels were exposed to the air.

For Example 13, a 73.1 gram sample of Sol C4 (prepared and diafilteredand concentrated as described above, 27.9 wt. % oxide and 3 wt. % aceticacid) and 25.5 grams of Sol T2 (prepared and diafiltered andconcentrated as described above, 26.6 wt. % oxide and 2.9 wt. % aceticacid) was charged in to a 500 ml RB flask. Water (49.2 grams) wasremoved via rotary evaporation resulting in a viscous somewhat drymaterial. Ethanol (15.15 grams), acrylic acid (2.9 grams), HEMA (1.5gram) and DI water (0.55 gram) were added to the flask. The contentswere stirred overnight resulting is a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 3 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This resulted in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hour then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Examples 12A, 12B and 13 were removedseparately from the ethanol bath, weighed, placed individually insidesmall canvas pouches, and then stored briefly in another ethanol bathbefore being loaded into the 10-L extractor vessel. The wet weight ofsample of Example 12A was 21.7 grams. The wet weight of sample ofExample 12B was 16.6 grams. The wet weight of sample of Example 13 was20.9 grams. For extraction of both Examples 12A, 12B and 13, about 800ml of 200-proof ethanol was added to the 10-L extractor of alaboratory-scale supercritical fluid extractor. The canvas bagscontaining the wet zirconia-based gels were transferred from the ethanolbath into the 10-L extractor so that the wet gels were completelyimmersed in the liquid ethanol inside the jacketed extractor vessel,which was heated and maintained at 60° C. The Example 12A, 12B and 13samples were subjected to the same extraction process as described abovefor Examples 1 and 2 samples. Afterwards, the dry aerogels were removedfrom their canvas pouches, weighed, and transferred into individual 237ml glass jars packed with tissue paper for storage. The dry Example 12Aaerogel was semi-translucent with a bluish tint and weighed 11.9 grams,corresponding to an overall weight loss during the supercriticalextraction process of 45.2%. The dry Example 12B aerogel wassemi-translucent with a bluish tint and weighed 9.2 grams, correspondingto an overall weight loss during the supercritical extraction process of44.6%. The dry Example 13 aerogel was semi-translucent with a bluishtint and weighed 11.5 grams, corresponding to an overall weight lossduring the supercritical extraction process of 45%.

Organic Burnout and Pre-Sinter Process

The extracted Example 12A, 12B, and 13 aerogel samples from above wereremoved from their closed containers and dried for 1 hour in open airprior to being set on a bed of zirconia beads in an unglazed porcelaincrucible, covered with alumina fiberboard then fired in air according tothe following schedule in a high temperature furnace (“THERMOLYNE TYPE46200”): i—heat from 20° C. to 225° C. at 18° C./hr. rate; ii—hold at225° C. for 24 hours; iii—heat from 225° C. to 400° C. at 6° C./hr.rate; iv—heat from 400° C. to 600° C. at 18° C./hr. rate; v—heat from600° C. to 1090° C. at 120° C./hr. rate; and vi—cool down from 1090° C.to 20° C. at 600° C./hr. rate.

After firing the samples of Examples 12B and 13 were crack free. Thesamples of Example 12A, 12B and 13 were diced into about 1 mm or 2.5 mmthick wafers. The wafers were ion exchanged by first placing them in a118 ml glass jar containing distilled water at a depth of about 2.5 cmand then vacuum infiltrating. The water was replaced with about a 2.5 cmdepth of 1.0N NH₄OH and the wafers were soaked overnight for 16 hours orlonger. The NH₄OH was then poured off and the jar was filled withdistilled water. The wafers were soaked in the distilled water for 1hour. The water was then replaced with fresh distilled water. This stepwas repeated until the pH of the soak water was equal to that of freshdistilled water. The wafers were then dried at 90-125° C. for a minimumof 1 hour. The pre-sintered at 1090° C. aerogels of Example 12B and 13had 48.1 and 46.4 volume % of oxides, respectively. as determined bydividing the geometric density of the pre-sintered wafer by theArchimedes density of the sintered wafer and then multiplying by 100.

Sintering Process

Wafers were set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heatfrom 1090° C. to 1250° C. at 120° C./hr. rate; iii—hold at 1250° C. for2 hours; and iv—cool down from 1250° C. to 20° C. at 600° C./hr. rate.

One of the sintered wafers of Example 12A was analyzed using XRD. TheExample 12B and 13, 1 mm wafers were polished on both faces and the 2.5mm wafers were polished on one face using a 12 open face lapping machine(“LAPMASTER”) for all but the final polishing step. The biaxial flexuralstrength was measured on the 2.5 mm samples after polishing using thetest method above. The samples were all adhered to a sample plate andwere then ground flat using 20 micrometer diamond tile (“3M TRIZACTDIAMOND TILE”) at a speed of 30 rpm. The abrasive was then switched to 9micrometer diamond tile (“3M TRIZACT DIAMOND TILE”) and grindingcontinued at 30 rpm until the majority of the 20 micrometer scratcheswere removed. The abrasive was then switched to 3 micrometer diamondtile (“3M TRIZACT DIAMOND TILE”) and grinding continued at 30 rpm untilthe majority of the 9 micrometer scratches were removed. The finalpolish was done using Buehler polishing equipment comprised of anelectrically driven head (“VECTOR POWER HEAD”) and a grinder-polisher(“BETA GRINDER-POLISHER”) and 3 micrometer diamond suspension (“METADIDIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”)until the majority of the scratches were removed. The samples polishedon both faces were translucent and lines were distinct when the sampleswere placed directly on top of them and at a distance. The samplesappeared slightly reddish in color in transmitted light and appearedslightly bluish in color in reflected light. The Archimedes density andT/T_(L) were measured as described above. After strength testing a pieceof aerogel of Example 13 was set on a bed of zirconia beads in analumina crucible and thermally etched in air in a rapid temperaturefurnace (CM Furnaces Inc.,) as follows: i—heat from 20° C. to 1200° C.at 450° C./hr. rate; ii—hold at 1200° C. for 0.5 hour; and iii—cool from1200° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample of Example 13 as describedin the test method described above. The grain size was determined usingthe line intercept method described in the test method above.

The properties of the sintered wafers are given in Table 6, below.

TABLE 6 Pol- Archi- ished medes Thick- Pol- Grain Compo- Exam- Density,ness ished Size, Strength, sition ple g/cm³ mm T/T_(L) nm MPa (XRD) 12A6.00 [ZrO2(C) major a = 5.146] 12B 6.00 0.4 1.89 13 6.00 1.5 236 877

Example 14

For Example 14A and 14B 63.85 gram sample of Sol C4 (prepared anddiafiltered and concentrated as described above, 27.9 wt. % oxide and 3wt. % acetic acid) and 26.25 grams of Sol B1 (prepared and diafilteredand concentrated as described above, 35.8 wt. % oxide and 3.2 wt. %acetic acid) was charged in to a 500 ml RB flask. Water (41.6 grams) wasremoved via rotary evaporation resulting in a viscous somewhat drymaterial. Ethanol (15.1 grams), acrylic acid (2.9 grams), HEMA (1.5gram) and DI water (1.5 gram) were added to the flask. The contents werestirred overnight resulting is a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.15 gram) was addedand stirred until dissolved. The contents of the flask were then purgedwith N₂ gas for 3 minutes. The sample (translucent and low viscosity)was charged to cylindrical containers (29 mm diameter). Each containerwas about 18 ml in volume and each was sealed on both ends (very littleair gap was left between the top and liquid). The samples were allowedto stand for about 1 hour then placed in an oven to cure (50° C., 4hours). This resulted in a clear translucent blue gel. The gel wasremoved from the container and placed in a 473 ml wide mouth jar. Thejar was filled with ethanol (denatured). The sample was soaked for 24hours then the ethanol was replaced with fresh ethanol. The sample wassoaked for 24 hours then the ethanol was replaced with a third batch offresh ethanol. The sample was allowed to soak until the supercriticalextraction was done. The above manipulations were done minimizing theamount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Example 14 were removed separately from theethanol bath, weighed, placed individually inside small canvas pouches,and then stored briefly in another ethanol bath before being loaded intothe 10-L extractor vessel. The wet weight of Example 14A sample was 20.7grams. The wet weight of Example 14B sample was 16.9 grams. About 850 mlof 200-proof ethanol was added to the 10-L extractor of alaboratory-scale supercritical fluid extractor. The canvas bagscontaining the wet zirconia-based gels of Examples 14A and 14B weretransferred from the ethanol bath into the 10-L extractor so that thewet gels were completely immersed in the liquid ethanol inside thejacketed extractor vessel, which was heated and maintained at 60° C. TheExample 14A and 14B samples were subjected to the same extractionprocess as described above for Examples 1 and 2 samples. Afterwards, thedry aerogels were removed from their canvas pouches, weighed, andtransferred into individual 237 ml glass jars packed with tissue paperfor storage. The dry Example 14A aerogel was semi-translucent with abluish tint and weighed 11.5 grams, corresponding to an overall weightloss during the supercritical extraction process of 44.4%. The dryExample 14B aerogel was semi-translucent with a bluish tint and weighed9.5 grams, corresponding to an overall weight loss during thesupercritical extraction process of 43.8%.

Organic Burnout and Pre-Sinter Process

The extracted Example 14A and 14B aerogel samples from above wereremoved from their closed containers and immediately set on a bed ofzirconia beads in an unglazed porcelain crucible, covered with aluminafiberboard then fired in air according to the following schedule in aTHERMOLYNE Type 46200 high temperature furnace: i—heat from 20° C. to225° C. at 18° C./hr. rate; ii—hold at 225° C. for 24 hours; iii—heatfrom 225° C. to 400° C. at 6° C./hr. rate; iv—heat from 400° C. to 600°C. at 18° C./hr. rate; v—heat from 600° C. to 1090° C. at 120° C./hr.rate; and vi—cool down from 1090° C. to 20° C. at 600° C./hr. rate.

After firing the samples were crack free. The sample of Example 14Acylinder was diced into about 1 mm or 2 mm thick wafers. The samplewafers were ion exchanged by first placing them in a 118 ml glass jarcontaining distilled water at a depth of about 2.5 cm and then vacuuminfiltrating. The water was replaced with about a 2.5 cm depth of 1.0NNH₄OH and the wafers were soaked overnight for 16 hours or longer. TheNH₄OH was then poured off and the jar was filled with distilled water.The wafers were soaked in the distilled water for 1 hour. The water wasthen replaced with fresh distilled water. This step was repeated untilthe pH of the soak water was equal to that of fresh distilled water. Thewafers were then dried at 90-125° C. for a minimum of 1 hour. Thepre-sintered (at 1090° C.) aerogel of Example 14A had 44.3 volume % ofoxides, as determined by dividing the geometric density of thepre-sintered wafer by the Archimedes density of the sintered wafer andthen multiplying by 100.

Sintering Process

A 2 mm wafer was set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a LINDBERG/BLUE M 1700° C. crucible furnace model56724: i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heat from1090° C. to 1250° C. at 120° C./hr. rate; iii—hold at 1250° C. for 2hours; and iv—cool down from 1250° C. to 20° C. at 600° C./hr. rate.

The wafer was polished on both faces using Buehler polishing equipmentcomprised of an electrically driven head (“VECTOR POWER HEAD”) and agrinder-polisher (“BETA GRINDER-POLISHER”). The sample was ground flaton both sides using 30 micrometer diamond lapping Film (“3M DIAMONDLAPPING FILM 668X”). Then 9 micrometer diamond lapping film (“3M DIAMONDLAPPING FILM 668X”) was used on both sides until the majority of the 30micrometer scratches were removed. Next the sample was polished on bothsides using 6 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”), until themajority of the 9 micrometer scratches were removed. Finally the samplewas polished on both sides using 3 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”), until the majority of the 6 micrometer scratches were removed.The sample was translucent and lines were distinct when the sample wasplaced directly on top of them and at a distance. The sample appearedreddish in color in transmitted light and appeared bluish in color inreflected light. The Archimedes density and T/T_(L) were measured asdescribed above. The samples were set on a bed of zirconia beads in analumina crucible and thermally etched in air in a rapid temperaturefurnace (CM Furnaces Inc.) as follows: i—heat from 20° C. to 1200° C. at450° C./hr. rate; ii—hold at 1200° C. for 0.5 hour; and iii—cool from1200° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described in the test method above.

The sintered Example 14A sample had an Archimedes density of 6.01 g/cm³,a polished T/T_(L) of 2.4 at a polished thickness of 1 mm, and anaverage grain size of 336 nm.

Example 15-17

For Example 15, 38.2 grams of diafiltered and concentrated Sol C1 (35.6wt. % oxide and about 3.7 wt. % acetic acid) and MEEAA (0.4 gram) werecharged to a 500 ml RB flask and mixed. Methoxypropanol (25 grams),acrylic acid (1.4 gram) and HEMA (0.73 gram) were added to the flask.Water and methoxypropanol (32.6 grams) were removed via rotaryevaporation. 2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.07 gram)was added and stirred until dissolved. The contents of the flask werethen purged with N, gas for 3 minutes. The sample (translucent and lowviscosity) was charged to cylindrical containers (29 mm diameter). Eachcontainer was about 18 ml in volume and each was sealed on both ends(very little air gap was left between the top and liquid). The sampleswere allowed to stand for about 1 hour then placed in an oven to cure(50° C., 4 hours). This resulted in a clear translucent blue gel. Thegel was removed from the container and placed in a 473 ml wide mouthjar. The jar was filled with ethanol (denatured). The sample was soakedfor 24 hours then the ethanol was replaced with fresh ethanol. Thesample was soaked for 24 hours then the ethanol was replaced with athird batch of fresh ethanol. The sample was allowed to soak until thesupercritical extraction was done. The above manipulations were doneminimizing the amount of time the gel was exposed to the air.

For Example 16, 117.85 grams of diafiltered and concentrated Sol C2(23.1 wt. % oxide and 2.4 wt. % acetic acid) was charged to a 500 ml RBflask. Water (67.85 grams) was removed via rotary evaporation resultingin a viscous somewhat dry material. Ethanol (15.15 grams), acrylic acid(2.9 grams), acrylamide (0.9 gram) and DI water (1.2 gram) were added tothe flask. The contents were stirred overnight resulting is a fluidtranslucent sol. 2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.15gram) was added and stirred until dissolved. The contents of the flaskwere then purged with N₂ gas for 3 minutes. The sample (translucent andlow viscosity) was charged to cylindrical containers (29 mm diameter).Each container was about 18 ml in volume and each was sealed on bothends (very little air gap was left between the top and liquid). Thesamples were allowed to stand for about 1 hour then placed in an oven tocure (50° C., 4 hours). This results in a clear translucent blue gel.The gel was removed from the container and placed in a 473 ml wide mouthjar. The jar was filled with ethanol (denatured). The sample was soakedfor 24 hours then the ethanol was replaced with fresh ethanol. Thesample was soaked for 24 hours then the ethanol was replaced with athird batch of fresh ethanol. The sample was allowed to soak until thesupercritical extraction was done. The above manipulations were doneminimizing the amount of time the gel was exposed to the air.

For Examples 17A and 17B, 92.38 grams of diafiltered and concentratedSol C3 (29.5 wt. % oxide and 3.1 wt. % acetic acid) was charged to a 500ml RB flask. Water (42.4 grams) was removed via rotary evaporationresulting in a viscous somewhat dry material. Ethanol (15.1 grams),acrylic acid (2.9 grams), 1-vinyl-2-pyrrolidione (1.5 gram) were addedto the flask. The contents were stirred overnight resulting in a fluidtranslucent sol. 2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.15gram) was added and stirred until dissolved. The contents of the flaskwere then purged with N, gas for 3 minutes). The sample (translucent andlow viscosity) was charged to cylindrical containers (29 mm diameter).Each container was about 18 ml in volume and each was sealed on bothends (very little air gap was left between the top and liquid). Thesamples were allowed to stand for about 1 hour then placed in an oven tocure (50° C., 4 hours). This results in a clear translucent blue gel.The gel was removed from the container and placed in a 473 ml wide mouthjar. The jar was filled with ethanol (denatured). The sample was soakedfor 24 hours then the ethanol was replaced with fresh ethanol. Thesample was soaked for 24 hours then the ethanol was replaced with athird batch of fresh ethanol. The sample was allowed to soak until thesupercritical extraction was done. The above manipulations were doneminimizing the amount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 15 was removed from themethoxypropanol bath, weighed, placed inside a small canvas pouch, andthen stored briefly in another methoxypropanol bath. The wet weight ofExample 15 sample was 25.1 grams. About 735 ml of methoxypropanol wasadded to the 10-L extractor of a laboratory-scale supercritical fluidextractor unit designed by and obtained from Thar Process, Inc.,Pittsburgh, Pa. The canvas bag containing the wet zirconia-based gel wastransferred from the methoxypropanol bath into the 10-L extractor sothat the wet gels were completely immersed in the liquid methoxypropanolinside the jacketed extractor vessel, which was heated and maintained at60° C. After the extractor vessel lid was sealed in place, liquid carbondioxide was pumped by a chilled piston pump (setpoint: 12.5° C.) througha heat exchanger to heat the CO₂ to 60° C. and into the 10-L extractorvessel until an internal pressure of 13.3 MPa was reached. At theseconditions, carbon dioxide is supercritical. Once the extractoroperating conditions of 13.3 MPa and 60° C. were met, a PID-controlledneedle valve regulated the pressure inside the extractor vessel byopening and closing to allow the extractor effluent to pass through aporous 316L stainless steel frit (obtained from Mott Corporation, NewBritain, Conn., under model #1100S-5.480 DIA-.062-10-A), then through aheat exchanger to cool the effluent to 30° C., and finally into a 5-Lcyclone separator vessel that was maintained at room temperature andpressure less than 5.5 MPa, where the extracted methoxypropanol andgas-phase CO₂ were separated and collected throughout the extractioncycle for recycling and reuse. Supercritical carbon dioxide (scCO₂) waspumped continuously through the 10-L extractor vessel for 7 hours fromthe time the operating conditions were achieved. After the 7-hourextraction cycle, the extractor vessel was slowly vented into thecyclone separator over 16 hours from 13.3 MPa to atmospheric pressure at60° C. before the lid was opened and the dried canvas pouch containingthe Example 15 aerogel was removed. The dry aerogel was removed from itscanvas pouch, weighed, and transferred into a 237 ml glass jar packedwith tissue paper for storage. The dry Example 15 aerogel wassemi-translucent with a bluish tint and weighed 14.3 g, corresponding toan overall weight loss during the supercritical extraction process of43%.

The wet ZrO₂-based gels of Examples 16 and 17 were removed separatelyfrom the ethanol bath, weighed, placed individually inside small canvaspouches, and then stored briefly in another ethanol bath before beingloaded into the 10-L extractor vessel. The wet weight of Example 16sample was 21.4 grams. The wet weight of Example 17A sample was 19grams. The wet weight of Example 17B sample was 21.2 grams. Forextraction of both Examples 16 and 17, about 800 ml of 200-proof ethanolwas added to the 10-L extractor of a laboratory-scale supercriticalfluid extractor. The canvas bags containing the wet zirconia-based gelswere transferred from the ethanol bath into the 10-L extractor so thatthe wet gels were completely immersed in the liquid ethanol inside thejacketed extractor vessel, which was heated and maintained at 60° C. TheExample 16 and 17 samples were subjected to the same extraction processas described above for Examples 1 and 2 samples. Afterwards, the dryaerogels were removed from their canvas pouches, weighed, andtransferred into individual 237 ml glass jars packed with tissue paperfor storage. The dry Example 16 aerogel was semi-translucent with abluish tint and weighed 11.6 grams, corresponding to an overall weightloss during the supercritical extraction process of 45.8%. The dryExample 17A aerogel was semi-translucent with a bluish tint and weighed10.4 grams, corresponding to an overall weight loss during thesupercritical extraction process of 45.3%. The dry Example 17B aerogelwas semi-translucent with a bluish tint.

Organic Burnout and Pre-Sinter Process

The extracted Example 15 sample was set on alumina fiberboard supportsin an unglazed porcelain crucible, covered with an alumina fiberboardthen fired in air according to the following schedule in a THERMOLYNEType 46200 high temperature furnace: i—heat from 20° C. to 225° C. at18° C./hr. rate; ii—hold at 225° C. for 24 hours; iii—heat from 225° C.to 400° C. at 6° C./hr. rate; iv—heat from 400° C. to 600° C. at 18°C./hr. rate; and v—cool down from 600° C. to 20° C. at 600° C./hr. rate.

After organic burnout a piece of the sample was set on an aluminafiberboard support in an alumina crucible, covered with an aluminafiberboard then fired in air according to the following schedule in acrucible furnace (Model 56724; “LINDBERG/BLUE M 1700° C.”): i—heat from20° C. to 665° C. at 600° C./hr. rate; ii—heat from 665° C. to 1090° C.at 120° C./hr. rate; and iii—cool down from 1090° C. to 20° C. at 600°C./hr. rate.

The fired sample was diced into about 1 mm thick wafers. The wafers wereion exchanged by first placing them in a 118 ml glass jar containingdistilled water at a depth of about 2.5 cm and then vacuum infiltrating.The water was replaced with about a 2.5 cm depth of 1.0N NH₄OH and thewafers were soaked overnight for 16 hours or longer. The NH₄OH was thenpoured off and the jar was filled with distilled water. The wafers weresoaked in the distilled water for 1 hour. The water was then replacedwith fresh distilled water. This step was repeated until the pH of thesoak water was equal to that of fresh distilled water. The wafers werethen dried at 90-125° C. for a minimum of 1 hour.

The Example 16 sample from above was placed in a 118 ml glass jar with alid. The lid had a 6.35 mm diameter hole in the top in order to achieveslow drying of the sample. The sample was dried in this way for 402hours and had a weight loss of 2%. The sample was set on a bed ofzirconia beads in an unglazed porcelain crucible, covered with analumina fiberboard then fired in air according to following schedule ina high temperature furnace (“THERMOLYNE TYPE 46200): i—heat from 20° C.to 225° C. at 18° C./hr. rate; ii—hold at 225° C. for 24 hours; iii—heatfrom 225° C. to 400° C. at 6° C./hr. rate; iv—heat from 400° C. to 600°C. at 18° C./hr. rate; v—heat from 600° C. to 1090° C. at 120° C./hr.rate; and vi—cool down from 1090° C. to 20° C. at 600° C./hr. rate.

After firing the sample was crack free. The cylinder was diced intoabout 1 mm thick wafers. The wafers were ion exchanged by first placingthem in a 118 ml glass jar containing distilled water at a depth ofabout 2.5 cm and then vacuum infiltrating. The water was replaced withabout a 2.5 cm depth of 1.0N NH₄OH and the wafers were soaked overnightfor about 17 hours. The NH₄OH was then poured off and the jar was filledwith distilled water. The wafers were soaked in the distilled water for1 hour. The water was then replaced with fresh distilled water. Thisstep was repeated until the pH of the soak water was equal to that offresh distilled water. The wafers were then dried at 90-125° C. for aminimum of 1 hour.

The extracted Example 17A aerogel sample from above was analyzed todetermine the BET surface area, pore size and porosity. The aerogel ofExample 17A had 274 M²/g of surface area MBET, 0.820 cm³/g of total porevolume and 120 A of average pore diameter.

The extracted Example 17B aerogel sample from above was removed from itsclosed container and placed in a 118 ml glass jar with a lid. The lidhad a 6.35 mm diameter hole in the top in order to achieve slow dryingof the sample. The sample was dried in this way for 1 hours prior tobeing set on a bed of zirconia beads in an unglazed porcelain crucible,covered with alumina fiberboard then fired in air according to thefollowing schedule in a high temperature furnace (“THERMOLYNE Type46200”): i—heat from 20° C. to 225° C. at 18° C./hr. rate; ii—hold at225° C. for 24 hours; iii—heat from 225° C. to 400° C. at 6° C./hr.rate; iv—heat from 400° C. to 600° C. at 18° C./hr. rate; v—heat from600° C. to 1090° C. at 120° C./hr. rate; and vi—cool down from 1090° C.to 20° C. at 600° C./hr. rate.

After firing the sample was crack free. The cylinder was diced intoabout 2.5 mm thick wafers. The Example 17B sample wafers were ionexchanged by first placing them in a 118 ml glass jar containingdistilled water at a depth of about 2.5 cm and then vacuum infiltrating.The water was replaced with about a 2.5 cm depth of 1.0N NH₄OH and thewafers were soaked overnight for 16 hours or longer. The NH₄OH was thenpoured off and the jar was filled with distilled water. The wafers weresoaked in the distilled water for 1 hour. The water was then replacedwith fresh distilled water. This step was repeated until the pH of thesoak water was equal to that of fresh distilled water. The wafers werethen dried at 90° C. for a minimum of 1 hour. The aerogel of Example 16had 11.65 volume % of oxides while the pre-sintered at 1090° C. aerogelof Example 16 had 53.1 volume % of oxides. The volume percent oxidevalues were calculated using the method described above. Thepre-sintered at 1090° C. aerogel of Example 17B had 47.9 volume % ofoxides, as determined by dividing the geometric density of thepre-sintered wafer by the Archimedes density of the sintered wafer andthen multiplying by 100.

Sintering Process

An Example 15 sample wafer was set on a bed of zirconia beads in analumina crucible, covered with alumina fiberboard then sintered in airaccording to the following schedule in a crucible furnace (Model 56724;“LINDBERG/BLUE M 1700° C.”): i—heat from 20° C. to 1090° C. at 600°C./hr. rate; ii—heat from 1090° C. to 1210° C. at 120° C./hr. rate;iii—hold at 1210° C. for 2 hours; and iv—cool down from 1210° C. to 20°C. at 600° C./hr. rate.

The sample had a yellowish brown color. The as fired wafer was analyzedby XRD. One face of the wafer was polished using polishing equipmentcomprised of an electrically driven head (“VECTOR POWER HEAD”) and agrinder-polisher (“BETA GRINDER-POLISHER”). The sample was ground flatusing 30 micrometer diamond lapping film (“3M DIAMOND LAPPING FILM668X”). Then 9 micrometer diamond lapping film (“3M DIAMOND LAPPING FILM668X”) was used until the majority of the 30 micrometer scratches wereremoved. Next the sample was polished using 6 micrometer diamondsuspension (“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMETPOLISHING CLOTH”) until the majority of the 9 micrometer scratches wereremoved. Finally the sample was polished using 3 micrometer diamondsuspension (“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMETPOLISHING CLOTH”) until the majority of the 6 micrometer scratches wereremoved. The polished wafer was set on a bed of zirconia beads in analumina crucible and thermally etched in air in a rapid temperaturefurnace (CM Furnaces Inc.) as follows: i—heat from 20° C. to 1160° C. at450° C./hr. rate; ii—hold at 1160° C. for 0.5 hour; and iii—cool from1160° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described in the test methods above.

One of the dried Example 16 wafers was set on a bed of zirconia beads inan alumina crucible, covered with an alumina fiberboard then sintered inair according to following schedule in a LINDBERG/BLUE M 1700° C.Crucible Furnace model 56724 obtained from Thermo Fischer Scientific,Waltham, Mass.: i—heat from 20° C. to 1090° C. at 600° C./hr. rate,ii—heat from 1090° C. to 1250° C. at 120° C./hr. rate, iii—hold at 1250°C. for 2 hours. iv—cool down from 1250° C. to 20° C. at 600° C./hr.rate.

The fired sample was transparent and colorless. The sample was polishedusing Buehler polishing equipment comprised of an electrically drivenhead (“VECTOR POWER HEAD” and a grinder-polisher (“BETAGRINDER-POLISHER”). The sample was ground flat on both sides using 30micrometer diamond lapping film (“3M DIAMOND LAPPING FILM 668X”). Then 9micrometer diamond lapping film (“3M DIAMOND LAPPING FILM 668X”) wasused on both sides until the majority of the 30 micrometer scratcheswere removed. Next the sample was polished on both sides using 6micrometer diamond suspension (“METADI DIAMOND SUSPENSION”) on apolishing cloth (“TEXMET POLISHING CLOTH”) until the majority of the 9micrometer scratches were removed. Finally the sample was polished onboth sides using 3 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 6 micrometer scratches were removed. The fired andpolished wafer is translucent and lines were distinct when the samplewas placed directly on top of them. The Archimedes density and T/T_(L)were measured as described above.

Wafers of the Example 17B sample were set on a bed of zirconia beads inan alumina crucible, covered with alumina fiberboard then sintered inair according to the following schedule in a crucible furnace (Model56724; “LINDBERG/BLUE M 1700° C.”): heat from 20° C. to 1090° C. at 600°C./hr. rate; heat from 1090° C. to 1250° C. at 120° C./hr. rate; hold at1250° C. for 2 hours; and iv—cool down from 1250° C. to 20° C. at 600°C./hr. rate.

The 2.5 mm wafers were polished on one face using a 12 open face lappingmachine (“LAPMASTER”) for all but the final polishing step. The biaxialflexural strength was measured on the 2.5 mm samples after polishingusing the test method above. The samples were all adhered to a sampleplate and were then ground flat using a 20 micrometer diamond tile (“3MTRIZACT DIAMOND TILE”) at a speed of 30 rpm. The abrasive was thenswitched to a 9 micrometer diamond tile (“3M TRIZACT DIAMOND TILE”) andgrinding continued at 30 rpm until the majority of the 20 micrometerscratches were removed. The abrasive was then switched to a 3 micrometerdiamond tile (“3M TRIZACT DIAMOND TILE”) and grinding continued at 30rpm until the majority of the 9 micrometer scratches were removed. Thefinal polish was done using Buehler polishing equipment comprised of anelectrically driven head (“VECTOR POWER HEAD”) and a grinder-polisher(“BETA GRINDER-POLISHER”) and 3 micrometer diamond suspension (“METADIDIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”)until the majority of the scratches were removed. After strength testinga piece of aerogel Example 17B was set on a bed of zirconia beads in analumina crucible and thermally etched in air in a rapid temperaturefurnace (CM Furnaces Inc.) as follows: i—heat from 20° C. to 1200° C. at450° C./hr. rate; ii—hold at 1200° C. for 0.5 hour; and iii—cool from1200° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described in the test methods above.

One of the Example 17B biaxial flexural strength sample fragments wassubmitted for XRD analysis.

The properties of the sintered wafers are given in Table 7, below.

TABLE 7 Pol- Archi- ished medes Thick- Pol- Grain Compo- Exam- Density/ness, ished Size, Strength, sition ple g/cm³ mm T/T_(L) nm MPa (XRD) 15262 [ZrO2(C) major a = 5.138] 16 5.98 0.47 1.97 17B 5.96 1.71 497 340[ZrO2(C) a = 5.15]

Example 18

For Example 18, 83.1 grams of diafiltered and concentrated Sol C3 (29.5wt. % oxide and 3.15 wt. % Acetic acid) was charged to 500 ml RB flask.Water (42.45 grams) was removed via rotary evaporation resulting in aviscous somewhat dry material. Ethanol (15.2 grams), acrylic acid (2.9grams), HEMA (1.5 gram) were added to the flask. The contents werestirred overnight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.15 gram) was addedand stirred until dissolved. The contents of the flask were then purgedwith N2 gas for 3 minutes. The sample (translucent and low viscosity)was charged to cylindrical containers (29 mm diameter). Each containerwas about 18 ml in volume and each was sealed on both ends (very littleair gap was left between the top and liquid). The samples were allowedto stand for about 1 hour then placed in an oven to cure (50° C., 4hours). This results in a clear translucent blue gel. The gel wasremoved from the container and placed in a 473 ml wide mouth jar. Thejar was filled with ethanol (denatured). The sample was soaked for 24hours then the ethanol was replaced with fresh ethanol. The sample wassoaked for 24 hours then the ethanol was replaced with a third batch offresh ethanol. The sample was allowed to soak until the supercriticalextraction was done. The above manipulations were done minimizing theamount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 18 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 18 was 11.6 grams. About 765 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 18 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 18 aerogel was semi-translucent with a bluishtint and weighed 6.6 g, corresponding to an overall weight loss duringthe supercritical extraction process of 43.1%.

Organic Burnout and Pre-Sinter Process

The extracted Example 18 aerogel sample from above was removed from itsclosed container and dried for 1 hour in open air prior to being set ona bed of zirconia beads in an unglazed porcelain crucible, covered withalumina fiberboard then fired in air according to the following schedulein a high temperature furnace (“THERMOLYNE TYPE 46200”): i—heat from 20°C. to 225° C. at 18° C./hr. rate; ii—hold at 225° C. for 24 hours;iii—heat from 225° C. to 400° C. at 6° C./hr. rate; iv—heat from 400° C.to 600° C. at 18° C./hr. rate; v—heat from 600° C. to 1090° C. at 120°C./hr. rate; and vi—cool down from 1090° C. to 20° C. at 600° C./hr.rate.

After firing the sample was crack free. The cylinder was diced intoabout 2 mm thick wafers. The wafers were ion exchanged by first placingthem in a 118 ml glass jar containing distilled water at a depth ofabout 2.5 cm and then vacuum infiltrating. The water was replaced withabout a 2.5 cm depth of 1.0N NH₄OH and the wafers were soaked overnightfor 16 hours or longer. The NH₄OH was then poured off and the jar wasfilled with distilled water. The wafers were soaked in the distilledwater for 1 hour. The water was then replaced with fresh distilledwater. This step was repeated until the pH of the soak water was equalto that of fresh distilled water. The wafers were then dried at 90-125°C. for a minimum of 1 hour. The pre-sintered (at 1090° C.) aerogel ofExample 18 had 47.9 volume % of oxides, as determined by dividing thegeometric density of the pre-sintered wafer by the Archimedes density ofthe sintered wafer and then multiplying by 100.

Sintering Process

The wafer was set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heatfrom 1090° C. to 1250° C. at 120° C./hr. rate; iii—hold at 1250° C. for2 hours; and iv—cool down from 1250° C. to 20° C. at 600° C./hr. rate.The Archimedes density was measured to be 6 g/cm³ as described in theabove procedure.

The sintered wafer of Example 18 was polished on both faces usingBuehler polishing equipment comprised of an electrically driven head(“VECTOR POWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”).First the sample was ground flat using a 45 micrometer metal bondeddiamond disc (Part No: 156145 from Buehler). Then 30 micrometer diamondlapping film (“3M DIAMOND LAPPING FILM 668X”) was used until themajority of the 45 micrometer scratches were removed. Then 9 micrometerdiamond lapping film 3M DIAMOND LAPPING FILM 668X”) was used until themajority of the 30 micrometer scratches were removed. Next the samplewas polished using 3 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 9 micrometer scratches were removed. Finally the samplewas polished using 0.25 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) (bothobtained from Buehler, Lake Bluff, Ill.) until the majority of the 3micrometer scratches were removed. The wafer was mounted in a lappingfixture (Model 150 from South Bay Technology, Inc.) during grinding andpolishing to maintain flat and parallel faces. The wafer was bonded tothe lapping fixture using hot-melt adhesive (“QUICKSTICK 135”). One sideof the wafer was ground and polished, then the wafer was remounted andthe other side was ground and polished.

The total transmittance was 61.5%, the diffuse transmittance was 11.8%,and the haze was 19.1%, measured using the spectrophotometer proceduredescribed earlier. The TLT and DLT spectra are designated in FIGS. 2 and3 as 1018 and 1118, respectively. The sample thickness was 1.01 mm.

Example 19

For Example 19A, 19B, and 19C a 24.4 gram sample of Sol C4 (prepared anddiafiltered and concentrated as described above, 27.9 wt. % oxide and 3wt. % acetic acid) and 76.6 grams of Sol T2 (prepared and diafilteredand concentrated as described above, 26.6 wt. % oxide and 2.9 wt. %acetic acid) was charged in to a 500 mL RB flask. Water (52.5 gram) wasremoved via rotary evaporation resulting in a viscous somewhat drymaterial. Ethanol (15.1 gram), acrylic acid (2.9 gram), HEMA (1.5 gram)and DI water (1.5 gram) were added to the flask. The contents werestirred overnight resulting is a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.15 gram) was addedand stirred until dissolved. The contents of the flask were then purgedwith N₂ gas for 3 minutes). The sample (translucent and low viscosity)was charged to cylindrical containers (29 mm diameter). Each containerwas about 18 mL in volume and each was sealed on both ends (very littleair gap was left between the top and liquid). The samples were allowedto stand for about 1 hour then placed in an oven to cure (50° C., 4hours). This resulted in a clear translucent blue gel. The gels wereremoved from their containers and placed in a 473 mL wide mouth jar. Thejar was filled with ethanol (denatured). The samples were soaked for 24hours then the ethanol was replaced with fresh ethanol. The samples weresoaked for 24 hours then the ethanol was replaced with a third batch offresh ethanol. The samples were allowed to soak until the supercriticalextraction was done. The above manipulations were done minimizing theamount of time the gels were exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Example 19A, 19B, and 19C were removed fromthe ethanol bath, weighed, placed inside small canvas pouches, and thenstored briefly in another ethanol bath before being loaded into the 10-Lextractor vessel. The wet weight of Example 19A was 20.8 grams. The wetweight of Example 19B was 19.5 grams. The wet weight of Example 19C was20.3 grams. About 735 mL of 200-proof ethanol was added to the 10-Lextractor of a laboratory-scale supercritical fluid extractor unit. Thecanvas bags containing the wet zirconia-based gels were transferred fromthe ethanol bath into the 10-L extractor so that the wet gels werecompletely immersed in the liquid ethanol inside the jacketed extractorvessel, which was heated and maintained at 60° C. The Example 19 sampleswere subjected to the same extraction process as described above forExamples 1 and 2 samples. Afterwards, the dry aerogels were removed fromtheir canvas pouches, weighed, and transferred into a 237 mL glass jarpacked with tissue paper for storage. The dry Example 19A aerogel wassemi-translucent with a bluish tint and weighed 11.2 grams,corresponding to an overall weight loss during the supercriticalextraction process of 46.2%. The dry Example 19B aerogel wassemi-translucent with a bluish tint and weighed 10.3 grams,corresponding to an overall weight loss during the supercriticalextraction process of 47.2%. The dry Example 19C aerogel wassemi-translucent with a bluish tint and weighed 10.9 grams,corresponding to an overall weight loss during the supercriticalextraction process of 46.3%.

Organic Burnout and Pre-Sinter Process

The extracted Example 19A, 19B, and 19C aerogel samples from above wereremoved from their closed containers and dried for 1 hour in open airprior to being set on a bed of zirconia beads in an unglazed porcelaincrucible, covered with alumina fiberboard then fired in air according tothe following schedule in a high temperature furnace (“THERMOLYNE TYPE46200”): i—heat from 20° C. to 225° C. at 18° C./hr rate; ii—hold at225° C. for 24 hours; iii—heat from 225° C. to 400° C. at 6° C./hr.rate; iv—heat from 400° C. to 600° C. at 18° C./hr. rate; v—heat from600° C. to 1090° C. at 120° C./hr. rate; and vi—cool down from 1090° C.to 20° C. at 600° C./hr. rate.

After firing the sample was crack free. The cylinder was diced intoabout 2 mm thick wafers. The wafers were ion exchanged by first placingthem in a 118 mL glass jar containing distilled water at a depth ofabout 2.5 cm and then vacuum infiltrating. The water was replaced withabout a 2.5 cm depth of 1.0N NH₄OH and the wafers were soaked overnightfor 16 hours or longer. The NH₄OH was then poured off and the jar wasfilled with distilled water. The wafers were soaked in the distilledwater for 1 hour. The water was then replaced with fresh distilledwater. This step was repeated until the pH of the soak water was equalto that of fresh distilled water. The wafers were then dried at 90-125°C. for a minimum of 1 hour. The pre-sintered at 1090° C. aerogels ofExample 19A, 19B and 19C had 46.4 volume % of oxides, as determined bydividing the geometric density of the pre-sintered wafer by theArchimedes density of the sintered wafer and then multiplying by 100.

Sintering Process

The wafers were set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heatfrom 1090° C. to 1250° C. at 120° C./hr. rate; iii—hold at 1250° C. for2 hours; and iv—cool down from 1250° C. to 20° C. at 600° C./hr. rate.The Archimedes density was measured to be 6.1 g/cm³ as described in theabove procedure.

A sintered wafer of Example 19C was polished on both faces using Buehlerpolishing equipment comprised of an electrically driven head (“VECTORPOWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”). First thesample was ground flat using a 45 micrometer metal bonded diamond disc(identified as Part No: 156145, Buehler). Then 30 micrometer diamondlapping film (“3M DIAMOND LAPPING FILM 668X”) was used until themajority of the 45 micrometer scratches were removed. Then 9 micrometerdiamond lapping film (“3M DIAMOND LAPPING FILM 668X”) was used until themajority of the 30 micrometer scratches were removed. Next the samplewas polished using 3 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 9 micrometer scratches were removed. Finally the samplewas polished using 0.25 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 3 micrometer scratches were removed. The wafer wasmounted in a lapping fixture (Model 150, South Bay Technology, Inc.)during grinding and polishing to maintain flat and parallel faces. Thewafer was bonded to the lapping fixture using a hot-melt adhesive(“QUICKSTICK 135”). One side of the wafer was ground and polished, thenthe wafer was remounted and the other side was ground and polished.

The total transmittance was 44.2%, the diffuse transmittance was 29.3%,and the haze was 66.2%, measured using the spectrophotometer proceduredescribed earlier. The TLT and DLT spectra are designated in FIGS. 2 and3 as 1019 and 1119, respectively. The sample thickness was 0.97 mm.

Wafers of Examples 19A and 19B were subject to the Hydrolytic StabilityTest and passed. The wafers of Examples 19A and 19B were subjected tothe 5 hour exposure to saturated steam at 135° C. under a pressure of0.2 MPa for up to five additional times. No phase transformation wasobserved during these hydrolytic stability tests at each of 5, 10, 15,and 30 hours of exposure.

Example 20

For Examples 20A, 20B, 20C, 20D, 20E, and 20F, a 96.9 gram sample of SolC4 (prepared and diafiltered and concentrated as described above, 27.9wt. % oxide and 3 wt. % acetic acid) and 102.2 grams of Sol T2 (preparedand diafiltered and concentrated as described above, 26.6 wt. % oxideand 2.9 wt. % acetic acid) was charged in to a 500 mL RB flask. Water(102.2 grams) was removed via rotary evaporation resulting in a viscoussomewhat dry material. Ethanol (30.3 grams), acrylic acid (5.8 grams),HEMA (2.9 grams) and DI water (3 grams) were added to the flask. Thecontents were stirred overnight resulting is a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.3 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 6 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 mL in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hr then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gels were removed fromthe containers and placed in a 473 mL wide mouth jar. The jar was filledwith ethanol (denatured). The samples were soaked for 24 hours then theethanol was replaced with fresh ethanol. The samples were soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The samples were allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gels were exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 20A, 20B, 20C, 20D, 20E, and 20F wereremoved from the ethanol bath, weighed, placed inside small canvaspouches, and then stored briefly in another ethanol bath before beingloaded into the 10-L extractor vessel. The wet weight of Example 20A was21 grams. The wet weight of Example 20B was 19.8 grams. The wet weightof Example 20C was 20.2 grams. The wet weight of Example 20D was 19grams. The wet weight of Example 20E was 18.1 grams. The wet weight ofExample 20F was 21 grams. About 855 mL of 200-proof ethanol was added tothe 10-L extractor of a laboratory-scale supercritical fluid extractorunit. The canvas bags containing the wet zirconia-based gels weretransferred from the ethanol bath into the 10-L extractor so that thewet gels were completely immersed in the liquid ethanol inside thejacketed extractor vessel, which was heated and maintained at 60° C. TheExample 20A, 20B, 20C, 20D, 20E, and 20F samples were subjected to thesame extraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogels were removed from their canvas pouches,weighed, and transferred into a 237 mL glass jar packed with tissuepaper for storage. The dry Example 20A aerogel was semi-translucent witha bluish tint and weighed 11.4 grams, corresponding to an overall weightloss during the supercritical extraction process of 45.7%. The dryExample 20B aerogel was semi-translucent with a bluish tint and weighed11 grams, corresponding to an overall weight loss during thesupercritical extraction process of 44.4%. The dry Example 20C aerogelwas semi-translucent with a bluish tint and weighed 11.1 grams,corresponding to an overall weight loss during the supercriticalextraction process of 45.1%. The dry Example 20D aerogel wassemi-translucent with a bluish tint and weighed 10.6 grams,corresponding to an overall weight loss during the supercriticalextraction process of 44.2%. The dry Example 20E aerogel wassemi-translucent with a bluish tint and weighed 10 grams, correspondingto an overall weight loss during the supercritical extraction process of44.8%. The dry Example 20F aerogel was semi-translucent with a bluishtint and weighed 11.6 grams, corresponding to an overall weight lossduring the supercritical extraction process of 44.8%.

Organic Burnout and Pre-Sinter Process

The extracted Example 20A, 20B, 20C, 20D, 20E, and 20F aerogel samplesfrom above were removed from their closed containers. The Example 20C,20E, and 20F samples were cracked. The Example 20A, 20B & 20C aerogelsamples were crack free and were set on a bed of zirconia beads in anunglazed porcelain crucible, covered with alumina fiberboard then firedin air according to the following schedule in a high temperature furnace(“THERMOLYNE TYPE 46200”): i—heat from 20° C. to 225° C. at 18° C./hr.rate; ii—hold at 225° C. for 24 hours; iii—heat from 225° C. to 400° C.at 6° C./hr. rate; iv—heat from 400° C. to 600° C. at 18° C./hr. rate;v—heat from 600° C. to 1090° C. at 120° C./hr. rate; and vi—cool downfrom 1090° C. to 20° C. at 600° C./hr. rate.

After firing the Example 20B and 20D samples were crack free. TheExample 20A sample was cracked. The Example 20B and 20D samples werediced into about 2 mm thick wafers. The wafers were ion exchanged byfirst placing them in a 118 mL glass jar containing distilled water at adepth of about 2.5 cm and then vacuum infiltrating. The water wasreplaced with about a 2.5 cm depth of 1.0N NH₄OH and the wafers weresoaked overnight for 16 hours or longer. The NH₄OH was then poured offand the jar was filled with distilled water. The wafers were soaked inthe distilled water for 1 hour. The water was then replaced with freshdistilled water. This step was repeated until the pH of the soak waterwas equal to that of fresh distilled water. The wafers were then driedat 90-125° C. for a minimum of 1 hour. The pre-sintered at 1090° C.aerogels of Example 20B and 20D had 44.4 volume % of oxides, asdetermined by dividing the geometric density of the pre-sintered waferby the Archimedes density of the sintered wafer and then multiplying by100.

Sintering Process

The wafers were set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heatfrom 1090° C. to 1250° C. at 120° C./hr. rate; iii—hold at 1250° C. for2 hours; and iv—cool down from 1250° C. to 20° C. at 600° C./hr. rate.The Archimedes density was measured to be 6.04 g/cm³ as described in theabove procedure.

A sintered wafer of Example 20D was polished on both faces using Buehlerpolishing equipment comprised of an electrically driven head (“VECTORPOWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”). First thesample was ground flat using a 45 micrometer metal bonded diamond disc(identified as Part No: 156145 from Buehler). Then 30 micrometer diamondlapping film (“3M DIAMOND LAPPING FILM 668X”) was used until themajority of the 45 micrometer scratches were removed. Then 9 micrometerdiamond lapping film (“3M DIAMOND LAPPING FILM 668X”) was used until themajority of the 30 micrometer scratches were removed. Next the samplewas polished using 3 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 9 micrometer scratches were removed. Finally the samplewas polished using 0.25 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 3 micrometer scratches were removed. The wafer wasmounted in a lapping fixture during grinding and polishing to maintainflat and parallel faces. The wafer was bonded to the lapping fixtureusing hot-melt adhesive (“QUICKSTICK 135”). One side of the wafer wasground and polished, then the wafer was remounted and the other side wasground and polished.

The total transmittance was 58.3%, the diffuse transmittance was 14.2%,and the haze was 24.3%, measured using the spectrophotometer proceduredescribed earlier. The TLT and DLT spectra are designated in FIGS. 2 and3 as 1020 and 1120, respectively. The sample thickness was 1.01 mm.

A wafer of Examples 20B was subject to the Hydrolytic Stability Test andpassed. The wafer of Example 20B was subjected to the 5 hour exposure tosaturated steam at 135° C. under a pressure of 0.2 MPa for up to fiveadditional times. No phase transformation was observed during thesehydrolytic stability tests at each of 5, 10, 15, and 30 hours ofexposure.

Example 21

For Example 21A, 21B, 21C, 21D, 21E, and 21F a 146.1 gram sample of SolC4 (prepared and diafiltered and concentrated as described above, 27.9wt. % oxide and 3 wt. % acetic acid) and 51.1 grams of Sol T2 (preparedand diafiltered and concentrated as described above, 26.6 wt. % oxideand 2.9 wt. % acetic acid) was charged in to a 500 mL RB flask. Water(100.2 grams) was removed via rotary evaporation resulting in a viscoussomewhat dry material. Ethanol (30.3 grams), acrylic acid (5.8 grams),HEMA (2.95 grams) and DI water (3 grams) were added to the flask. Thecontents were stirred overnight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.3 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 6 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 mL in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hr then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gels were removed fromthe containers and placed in a 473 mL wide mouth jar. The jar was filledwith ethanol (denatured). The samples were soaked for 24 hours then theethanol was replaced with fresh ethanol. The samples were soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The samples were allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gels were exposed to the air.

Extraction Process

The wet ZrO₂-based gels of Example 21A, 21B, 21C, 21D, 21E, and 21F wereremoved from the ethanol bath, weighed, placed inside small canvaspouches, and then stored briefly in another ethanol bath before beingloaded into the 10-L extractor vessel. The wet weight of Example 21A was21.8 grams. The wet weight of Example 21B was 20.4 grams. The wet weightof Example 21C was 20.9 grams. The wet weight of Example 21D was 20.9grams. The wet weight of Example 21E was 21.2 grams. The wet weight ofExample 21F was 14 grams. About 735 mL of 200-proof ethanol was added tothe 10-L extractor of a laboratory-scale supercritical fluid extractorunit. The canvas bags containing the wet zirconia-based gels weretransferred from the ethanol bath into the 10-L extractor so that thewet gels were completely immersed in the liquid ethanol inside thejacketed extractor vessel, which was heated and maintained at 60° C. TheExample 21A, 21B, 21C, 21D, 21E, and 21F samples were subjected to thesame extraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogels were removed from their canvas pouches,weighed, and transferred into a 237 mL glass jar packed with tissuepaper for storage. The dry Example 21A aerogel was semi-translucent witha bluish tint and weighed 11.9 grams, corresponding to an overall weightloss during the supercritical extraction process of 45.4%. The dryExample 21B aerogel was semi-translucent with a bluish tint and weighed11.1 grams, corresponding to an overall weight loss during thesupercritical extraction process of 45.6%. The dry Example 21C aerogelwas semi-translucent with a bluish tint and weighed 11.3 grams,corresponding to an overall weight loss during the supercriticalextraction process of 45.9%. The dry Example 21D aerogel was opaque andcracked and weighed 12.7 grams, corresponding to an overall weight lossduring the supercritical extraction process of 39.2%. The dry Example21E aerogel was opaque and cracked and weighed 12.7 grams, correspondingto an overall weight loss during the supercritical extraction process of40.1%. The dry Example 21F aerogel was opaque and cracked and weighed8.5 grams, corresponding to an overall weight loss during thesupercritical extraction process of 39.3%.

Organic Burnout and Pre-Sinter Process

The extracted Example 21A, 21B, and 21C aerogel samples from above wereremoved from their closed container and set on a bed of zirconia beadsin an unglazed porcelain crucible, covered with alumina fiberboard thenfired in air according to the following schedule in a high temperaturefurnace (“THERMOLYNE TYPE 46200”): i—heat from 20° C. to 225° C. at 18°C./hr. rate; ii—hold at 225° C. for 24 hours; iii—heat from 225° C. to400° C. at 6° C./hr. rate; iv—heat from 400° C. to 600° C. at 18° C./hr.rate; v—heat from 600° C. to 1090° C. at 120° C./hr. rate; and vi—cooldown from 1090° C. to 20° C. at 600° C./hr. rate.

After firing the samples were crack free. The cylinders were diced intoabout 2 mm thick wafers. The wafers were ion exchanged by first placingthem in a 118 mL glass jar containing distilled water at a depth ofabout 2.5 cm and then vacuum infiltrating. The water was replaced withabout a 2.5 cm depth of 1.0N NH₄OH and the wafers were soaked overnightfor 16 hours or longer. The NH₄OH was then poured off and the jar wasfilled with distilled water. The wafers were soaked in the distilledwater for 1 hour. The water was then replaced with fresh distilledwater. This step was repeated until the pH of the soak water was equalto that of fresh distilled water. The wafers were then dried at 90-125°C. for a minimum of 1 hour. The pre-sintered at 1090° C. aerogels ofExample 21A, 21B, and 21C had 46.6 volume % of oxides, as determined bydividing the geometric density of the pre-sintered wafer by theArchimedes density of the sintered wafer and then multiplying by 100.

Sintering Process

The Example 21A, 21B, and 21C wafers were set on a bed of zirconia beadsin an alumina crucible, covered with alumina fiberboard then sintered inair according to the following schedule in a crucible furnace (Model56724; “LINDBERG/BLUE M 1700° C.”): i—heat from 20° C. to 1090° C. at600° C./hr. rate; ii—heat from 1090° C. to 1250° C. at 120° C./hr. rate;iii—hold at 1250° C. for 2 hours; and iv—cool down from 1250° C. to 20°C. at 600° C./hr. rate. The Archimedes density was measured to be 6g/cm³ as described in the above procedure.

A sintered wafer of Example 21C was polished on both faces using Buehlerpolishing equipment comprised of an electrically driven head (“VECTORPOWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”). First thesample was ground flat using a 45 micrometer metal bonded diamond disc(identified as Part No: 156145 from Buehler). Then 30 micrometer diamondlapping film (“3M DIAMOND LAPPING FILM 668X”) was used until themajority of the 45 micrometer scratches were removed. Then 9 micrometerdiamond lapping film (“3M DIAMOND LAPPING FILM 668X”) was used until themajority of the 30 micrometer scratches were removed. Next the samplewas polished using 3 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 9 micrometer scratches were removed. Finally the samplewas polished using 0.25 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”) until themajority of the 3 micrometer scratches were removed. The wafer wasmounted in a lapping fixture during grinding and polishing to maintainflat and parallel faces. The wafer was bonded to the lapping fixtureusing hot-melt adhesive (“QUICKSTICK 135”). One side of the wafer wasground and polished, then the wafer was remounted and the other side wasground and polished.

The total transmittance was 65.2%, the diffuse transmittance was 8.9%,and the haze was 13.7%, measured using the spectrophotometer proceduredescribed earlier. The TLT and DLT spectra are designated in FIGS. 2 and3 as 1021 and 1121, respectively. The sample thickness was 1.00 mm.

Wafers of Examples 21A and 21C were subject to the Hydrolytic StabilityTest and passed. The wafers of Examples 21A and 21C were subjected tothe 5 hour exposure to saturated steam at 135° C. under a pressure of0.2 MPa for up to five additional times. No phase transformation wasobserved during these hydrolytic stability tests at each of 5, 10, 15,and 30 hours of exposure.

Example 22

For Example 22, 108.2 grams of diafiltered and concentrated Sol A1 (25.6wt. % oxide and 2.3 wt. % acetic acid) was charged to 500 ml RB flask.Water (58.2 grams) was removed via rotary evaporation resulting in aviscous somewhat dry material. Ethanol (15.2 grams), acrylic acid (2.9grams), and HEMA (1.5 gram) were added to the flask. The contents werestirred overnight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram) was added andstirred until dissolved. The contents of the flask were then purged withN2 gas for 3 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 22 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 22 was 22.1 grams. About 785 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 22 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 22 aerogel was semi-translucent with a bluishtint and weighed 12.1 grams, corresponding to an overall weight lossduring the supercritical extraction process of 45.7%.

Organic Burnout and Pre-Sinter Process

The extracted Example 22 aerogel sample from above was removed from itsclosed container and set on a bed of zirconia beads in an aluminacrucible, covered with alumina then fired in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 600° C. at 10° C./hr. rate; ii—heatfrom 600° C. to 1090° C. at 120° C./hr. rate; iii—hold at 1090° C. for 1hour; and iv—cool down from 1090° C. to 20° C. at 600° C./hr. rate.

After firing the sample was crack free. The cylinder was diced intoabout 1.8 mm thick wafers. The wafers were ion exchanged by firstplacing them in a 118 ml glass jar containing distilled water at a depthof about 2.5 cm and then vacuum infiltrating. The water was replacedwith about a 2.5 cm depth of 1.0N NH₄OH and the wafers were soakedovernight for 16 hours or longer. The NH₄OH was then poured off and thejar was filled with distilled water. The wafers were soaked in thedistilled water for 1 hour. The water was then replaced with freshdistilled water. This step was repeated until the pH of the soak waterwas equal to that of fresh distilled water. The wafers were then driedat 40° C.

Sintering Process

The wafer was set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heatfrom 1090° C. to 1250° C. at 120° C./hr. rate; iii—hold at 1250° C. for2 hours; and iv—cool down from 1250° C. to 20° C. at 600° C./hr. rate.

The sintered wafer of Example 22 was polished on both faces usingBuehler polishing equipment comprised of an electrically driven head(“VECTOR POWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”).First the sample was ground flat using a 45 micrometer metal bondeddiamond disc (identified as Part No: 156145 from Buehler). Then 30micrometer diamond lapping film (“3M DIAMOND LAPPING FILM 668X”) wasused until the majority of the 45 micrometer scratches were removed.Then 9 micrometer diamond lapping film (“3M DIAMOND LAPPING FILM 668X”)was used until the majority of the 30 micrometer scratches were removed.Next the sample was polished using 3 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 9 micrometer scratches were removed.Finally the sample was polished using 0.25 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 3 micrometer scratches were removed.The wafer was mounted in a lapping fixture during grinding and polishingto maintain flat and parallel faces. The wafer was bonded to the lappingfixture using hot-melt adhesive (“QUICKSTICK 135”). One side of thewafer was ground and polished, then the wafer was remounted and theother side was ground and polished.

The total transmittance was 34.7%, the diffuse transmittance was 31.8%,and the haze was 95.3%, measured using the spectrophotometer proceduredescribed earlier. The TLT and DLT spectra are designated in FIGS. 2 and3 as 1022 and 1122, respectively. The sample thickness was 1.01 mm.

Example 23

To prepare Example 23, Sol S4 was diafiltered and concentrated asdescribed above for Sol T1. The resulting sol was 54.9 wt. % ZrO₂/Y₂O₃and about 5.5 wt. % acetic acid. The sol (100.24 grams) was charged to a500 ml round bottom (RB) flask. Ethanol (30 grams), acrylic acid (5.75grams), and HEMA (4.5 grams) were added to the flask.2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.4 gram) was added andthe contents stirred for 4 hours. The contents of the flask were thenpurged with N, gas for 6 minutes). The sample (translucent and lowviscosity) was charged to cylindrical containers (29 mm diameter). Eachcontainer was about 18 ml in volume and each was sealed on both ends(very little air gap was left between the top and liquid). The sampleswere allowed to stand about 1 hour then placed in an oven to cure (50°C., 4 hours). This results in a clear translucent blue gel. The gel wasremoved from the container and placed in a 473 ml wide mouth jar. Thejar was filled with ethanol (denatured). The sample was soaked for 24hour then the ethanol was replaced with fresh ethanol. The sample wassoaked for 24 hours then the ethanol was replaced with a third batch offresh ethanol. The sample was allowed to soak until the supercriticalextraction was done. The above manipulations were done minimizing theamount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 23 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 23 was 19.6 grams. About 700 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 23 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 23 aerogel was semi-translucent with a bluishtint and weighed 9.9 grams, corresponding to an overall weight lossduring the supercritical extraction process of about 50%.

Organic Burnout and Pre-Sinter Process

The extracted Example 23 aerogel sample from above was removed from itsclosed container and set on a bed of zirconia beads in an aluminacrucible, covered with alumina then fired in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 600° C. at 10° C./hr. rate ii—heatfrom 600° C. to 1000° C. at 120° C./hr. rate; iii—hold at 1000° C. for 1hour; and iv—cool down from 1000° C. to 20° C. at 600° C./hr. rate.

After firing the sample was crack free. The cylinder was ion exchangedby first placing it in a 118 ml glass jar containing distilled water ata depth of about 2.5 cm and then vacuum infiltrating. The water wasreplaced with about a 2.5 cm depth of 1.0N NH₄OH and the cylinder wassoaked overnight for 16 hours or longer. The NH₄OH was then poured offand the jar was filled with distilled water. The cylinder was soaked inthe distilled water for 1 hour. The water was then replaced with freshdistilled water. This step was repeated until the pH of the soak waterwas equal to that of fresh distilled water. The cylinder was then driedat 60° C. overnight.

The cylinder was diced into about 1.8 mm thick wafers. The wafers weredried at 90-125° C.

Sintering Process

The wafer was set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1000° C. at 600° C./hr. rate; ii—heatfrom 1000° C. to 1225° C. at 120° C./hr. rate; iii—hold at 1225° C. for2 hours; iv—cool down from 1225° C. to 20° C. at 600° C./hr. rate.

The sintered wafer of Example 23 was polished on both faces usingBuehler polishing equipment comprised of an electrically driven head(“VECTOR POWER HEAD”) and a grinder-polisher (“BETA GRINDER-POLISHER”).First the sample was ground flat using a 45 micrometer metal bondeddiamond disc (identified as Part No: 156145 from Buehler). Then 30micrometer diamond lapping film (“3M DIAMOND LAPPING FILM 668X”) wasused until the majority of the 45 micrometer scratches were removed.Then 9 micrometer diamond lapping film (“3M DIAMOND LAPPING FILM 668X”)was used until the majority of the 30 micrometer scratches were removed.Finally the sample was polished using 3 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”) until the majority of the 9 micrometer scratches were removed.The final wafers were about 13 mm in diameter and 0.9 mm thick. TheT/T_(L) was measured to be 0.96 as described above. The average biaxialflexural strength was measured to be 1163 MPa using the test methoddescribed above. The grain size was measured to be 192 nm by FESEMexamination of the fracture surface and using the line intercept methodaccording to the methods described above.

Example 24

For Example 24, a 48.8 gram sample of Sol C4 (prepared and diafilteredand concentrated as described above, 27.9 wt. % oxide and 3 wt. % aceticacid) and 153.2 grams of Sol T2 (prepared and diafiltered andconcentrated as described above, 26.6 wt. % oxide and 2.55 wt. % aceticacid) was charged in to a 500 ml RB flask. Water (102.7 grams) wasremoved via rotary evaporation resulting in a viscous somewhat drymaterial. Ethanol (30.3 grams), acrylic acid (5.8 grams), HEMA (2.9grams), and DI water (0.7 gram) were added to the flask. The contentswere stirred overnight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.3 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 6 minutes. The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 24 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 24 was 20.2 grams. About 835 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 24 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 24 aerogel was semi-translucent with a bluishtint and weighed 11.1 grams, corresponding to an overall weight lossduring the supercritical extraction process of 45%.

Organic Burnout and Pre-Sinter Process

The extracted Example 24 aerogel sample prepared above was removed fromits closed container and set on a bed of zirconia beads in an unglazedporcelain crucible, covered with alumina fiberboard then fired in airaccording to the following schedule in a high temperature furnace(“THERMOLYNE Type 46200”): i—heat from 20° C. to 225° C. at 18° C./hr.rate; ii—hold at 225° C. for 24 hours; iii—heat from 225° C. to 400° C.at 6° C./hr. rate; iv—heat from 400° C. to 600° C. at 18° C./hr. ratev—heat from 600° C. to 1090° C. at 120° C./hr rate; and vi—cool downfrom 1090° C. to 20° C. at 600° C./hr. rate.

After firing the sample was crack free. The cylinder was diced intoabout 1 mm thick wafers. The Example 24 wafers were ion exchanged byfirst placing them in a 118 ml glass jar containing distilled water at adepth of about 2.5 cm and then vacuum infiltrating. The water wasreplaced with about a 2.5 cm depth of 1.0N NH₄OH and the wafers weresoaked overnight for 16 hours or longer. The NH₄OH was then poured offand the jar was filled with distilled water. The wafers were soaked inthe distilled water for 1 hour. The water was then replaced with freshdistilled water. This step was repeated until the pH of the soak waterwas equal to that of fresh distilled water. The wafers were then driedat 90-125° C. for a minimum of 1 hour.

The pre-sintered at 1090° C. aerogel of Example 24 had 46.5 volume % ofoxides, as determined by dividing the geometric density of thepre-sintered wafer by the Archimedes density of the sintered wafer andthen multiplying by 100.

Sintering Process

A wafer was set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heatfrom 1090° C. to 1250° C. at 120° C./hr. rate; iii—hold at 1250° C. for2 hours; and iv—Cool down from 1250° C. to 20° C. at 600° C./hr. rate.This same wafer was sintered again as above but with a hold at 1250° C.for 20 hours.

The sintered wafer was polished on both faces using Buehler polishingequipment comprised of an electrically driven head (“VECTOR POWER HEAD”)and a grinder-polisher (“BETA GRINDER-POLISHER”). The sample was groundflat on both sides using 30 micrometer diamond lapping film (“3M DIAMONDLAPPING FILM 668X”). Then 9 micrometer diamond lapping film (“3M DIAMONDLAPPING FILM 668X”) was used on both sides until the majority of the 30micrometer scratches were removed. Next the sample was polished on bothsides using 6 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”), until themajority of the 9 micrometer scratches were removed. Finally the samplewas polished on both sides using 3 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”), until the majority of the 6 micrometer scratches were removed.The Archimedes density and T/T_(L) were measured as described above. Thedisc was submitted for x-ray diffraction to determine the phasespresent.

After XRD was done the sample was set on a bed of zirconia beads in analumina crucible and thermally etched in air in a rapid temperaturefurnace (CM Furnaces Inc.) as follows: i—heat from 20° C. to 1200° C. at450° C./hr. rate; ii—hold at 1200° C. for 0.5 hour; and iii—cool from1200° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described in the test method above.

The properties of the sintered wafer of Example 24 are given in Table 8,below.

TABLE 8 Pol- Archi- ished Phase medes Thick- Pol- Grain Compo- Exam-Density, ness, ished Size, Strength, sition ple g/cm³ mm T/T_(L) nm MPa(XRD) 24 6.04 0.54 1.08 168 [ZrO₂(C1) 2% a = 5.39; ZrO₂(C2) 15% a =5.15; ZrO₂(T) 83% a = 3.61 c = 5.18]

Example 25

For Example 25, a 48.8 gram sample of Sol C4 (prepared and diafilteredand concentrated as described above, 27.9 wt. % oxide and 3 wt. % aceticacid) and 153.2 grams of Sol T2 (prepared and diafiltered andconcentrated as described above, 26.6 wt. % oxide and 2.55 wt. % aceticacid) was charged in to a 500 ml RB flask. Water (102.7 grams) wasremoved via rotary evaporation resulting in a viscous somewhat drymaterial. Ethanol (30.3 grams), acrylic acid (5.8 grams), HEMA (2.9grams) and DI water (0.7 gram) were added to the flask. The contentswere stirred overnight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.3 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas (6 minutes). The sample (translucent and low viscosity) wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hr then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 25 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 25 was 20.2 grams. About 835 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 25 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 25 aerogel was semi-translucent with a bluishtint and weighed 11.1 grams, corresponding to an overall weight lossduring the supercritical extraction process of 45%.

Organic Burnout and Pre-Sinter Process

The extracted Example 25 aerogel sample prepared above was removed fromits closed container and set on a bed of zirconia beads in an unglazedporcelain crucible, covered with alumina fiberboard then fired in airaccording to the following schedule in a high temperature furnace(“THERMOLYNE Type 46200”): i—heat from 20° C. to 225° C. at 18° C./hr.rate; ii—hold at 225° C. for 24 hours; iii—heat from 225° C. to 400° C.at 6° C./hr. rate; iv—heat from 400° C. to 600° C. at 18° C./hr. rate;v—heat from 600° C. to 1090° C. at 120° C./hr. rate; and vi—cool downfrom 1090° C. to 20° C. at 600° C./hr. rate.

After firing the sample was crack free. The cylinder was diced intoabout 1 mm thick wafers. The Example 25 wafers were ion exchanged byfirst placing them in a 118 ml glass jar containing distilled water at adepth of about 2.5 cm and then vacuum infiltrating. The water wasreplaced with about a 2.5 cm depth of 1.0N NH₄OH and the wafers weresoaked overnight for 16 hours or longer. The NH₄OH was then poured offand the jar was filled with distilled water. The wafers were soaked inthe distilled water for 1 hour. The water was then replaced with freshdistilled water. This step was repeated until the pH of the soak waterwas equal to that of fresh distilled water. The wafers were then driedat 90-125° C. for a minimum of 1 hour.

The pre-sintered at 1090° C. aerogel of Example 25 had 46.6 volume % ofoxides, as determined by dividing the geometric density of thepre-sintered wafer by the Archimedes density of the sintered wafer andthen multiplying by 100.

Sintering Process

A wafer was set on a bed of zirconia beads in an alumina crucible,covered with alumina fiberboard then sintered in air according to thefollowing schedule in a crucible furnace (Model 56724; “LINDBERG/BLUE M1700° C.”): i—heat from 20° C. to 1090° C. at 600° C./hr. rate; ii—heatfrom 1090° C. to 1500° C. at 120° C./hr. rate; iii—hold at 1500° C. for2 hours; and iv—Cool down from 1500° C. to 20° C. at 600° C./hr. rate.

The sintered wafer was polished on both faces using Buehler polishingequipment comprised of an electrically driven head (“VECTOR POWER HEAD”)and a grinder-polisher (“BETA GRINDER-POLISHER”). The sample was groundflat on both sides using 30 micrometer diamond lapping film (“3M DIAMONDLAPPING FILM 668X”). Then 9 micrometer diamond lapping film (“3M DIAMONDLAPPING FILM 668X”) was used on both sides until the majority of the 30micrometer scratches were removed. Next the sample was polished on bothsides using 6 micrometer diamond suspension (“METADI DIAMONDSUSPENSION”) on a polishing cloth (“TEXMET POLISHING CLOTH”), until themajority of the 9 micrometer scratches were removed. Finally the samplewas polished on both sides using 3 micrometer diamond suspension(“METADI DIAMOND SUSPENSION”) on a polishing cloth (“TEXMET POLISHINGCLOTH”), until the majority of the 6 micrometer scratches were removed.The Archimedes density and T/T_(L) were measured as described above. Thedisc was submitted for x-ray diffraction to determine the phasespresent.

After XRD was done the sample was set on a bed of zirconia beads in analumina crucible and thermally etched in air in a rapid temperaturefurnace (CM Furnaces Inc.) as follows: i—heat from 20° C. to 1200° C. at450° C./hr. rate; ii—hold at 1200° C. for 0.5 hour; and iii—cool from1200° C. to 20° C. at 600° C./hr. rate.

FESEM was done on the thermally etched sample as described in the testmethod described above. The grain size was determined using the lineintercept method described in the test method above.

The properties of the sintered wafer of Example 25 are given in Table 9,below.

TABLE 9 Pol- Archi- ished Phase medes Thick- Pol- Grain Compo- Exam-Density, ness, ished Size, Strength, sition ple g/cm³ mm T/T_(L) nm MPa(XRD) 25 6.05 0.57 1.00 444 [ZrO₂(C2) 50% a = 5.14; ZrO₂(T) 50% a = 3.60c = 5.18]

Example 26

To prepare Example 26, Sol S3 was diafiltered and concentrated asdescribed above for Sol T1. The resulting sol was 54.3 wt. % ZrO₂/Y₂O₃and 5.6 wt. % acetic acid. The sol (50 grams) was charged to a 500 ml RBflask. Ethanol (15.15 grams), acrylic acid (2.9 gram) and ethoxylatedpentaerythritol tetraacrylate (“SR454”) (1.5 gram) were added to theflask. 2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram) wasadded and the contents stirred to dissolve the2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”). The contents of theflask were then purged with N₂ gas for 3 minutes. The sample(translucent and low viscosity) was charged to cylindrical containers(29 mm diameter). Each container was about 18 ml in volume and each wassealed on both ends (very little air gap was left between the top andliquid). The samples were allowed to stand about 12 hours then placed inan oven to cure (50° C., 4 hours). This results in a clear translucentblue gel. The gel was removed from the container and placed in a 473 mlwide mouth jar. The jar was filled with ethanol (denatured). The samplewas soaked for 24 hours then the ethanol was replaced with freshethanol. The sample was soaked for 24 hours then the ethanol wasreplaced with a third batch of fresh ethanol. The sample was allowed tosoak until the supercritical extraction was done. The abovemanipulations were done minimizing the amount of time the gel wasexposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 26 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 26 was 21.4 grams. About 805 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 26 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 26 aerogel was semi-translucent with a bluishtint and weighed 11.2 grams, corresponding to an overall weight lossduring the supercritical extraction process of 47.7%.

Example 27

To prepare Example 27, Sol S3 was diafiltered and concentrated asdescribed above for Sol T1. The resulting sol was 54.2 wt. % ZrO₂/Y₂O₃and 5.6 wt. % acetic acid. The sol (50 grams) was charged to a 500 ml RBflask. Ethanol (15.1 grams), acrylic acid (2.9 gram) and polyethyleneglycol (400) dimethacrylate (“SR603”) (1.5 gram) were added to theflask. 2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram) wasadded and the contents stirred to dissolve the2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”). The contents of theflask were then purged with N, gas for 3 minutes. The sample(translucent and low viscosity) was charged to cylindrical containers(29 mm diameter). Each container was about 18 ml in volume and each wassealed on both ends (very little air gap was left between the top andliquid). The samples were allowed to stand about 12 hours then placed inan oven to cure (50° C., 4 hours). This results in a clear translucentblue gel. The gel was removed from the container and placed in a 473 mlwide mouth jar. The jar was filled with ethanol (denatured). The samplewas soaked for 24 hours then the ethanol was replaced with freshethanol. The sample was soaked for 24 hours then the ethanol wasreplaced with a third batch of fresh ethanol. The sample was allowed tosoak until the supercritical extraction was done. The abovemanipulations were done minimizing the amount of time the gel wasexposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 27 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 27 was 19.9 grams. About 765 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 27 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 27 aerogel was semi-translucent with a bluishtint and weighed 11.1 grams, corresponding to an overall weight lossduring the supercritical extraction process of 44.2%.

Example 28

To prepare Example 28, Sol S3 was diafiltered and concentrated asdescribed above for Sol T1. The resulting sol was 54.2 wt. % ZrO₂/Y₂O₃and 5.6 wt. % acetic acid. The sol (50 grams) was charged to 500 ml RBflask. Ethanol (15.1 grams), acrylic acid (2.9 grams) and ethoxylatedpentaerythritol tetraacrylate (“SR494”) (1.5 gram) were added to theflask. 2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram) wasadded and the contents stirred to dissolve the2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”). The contents of theflask were then purged with N₂ gas for 3 minutes. The sample(translucent and low viscosity) was charged to cylindrical containers(29 mm diameter). Each container was about 18 ml in volume and each wassealed on both ends (very little air gap was left between the top andliquid). The samples were allowed to stand about 12 hours then placed inan oven to cure (50° C., 4 hours). This results in a clear translucentblue gel. The gel was removed from the container and placed in a 473 mlwide mouth jar. The jar was filled with ethanol (denatured). The samplewas soaked for 24 hours then the ethanol was replaced with freshethanol. The sample was soaked for 24 hours then the ethanol wasreplaced with a third batch of fresh ethanol. The sample was allowed tosoak until the supercritical extraction was done. The abovemanipulations were done minimizing the amount of time the gel wasexposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 28 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 28 was 22.2 grams. About 765 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 28 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 28 aerogel was semi-translucent with a bluishtint and weighed 11.4 grams, corresponding to an overall weight lossduring the supercritical extraction process of 48.6%.

Example 29

To prepare Example 29, Sol S3 was diafiltered and concentrated asdescribed above for Sol T1. The resulting sol was 54.2 wt. % ZrO₂/Y₂O₃and 5.6 wt. % acetic acid. The sol (50 grams) was charged to 500 ml RBflask. Ethanol (15.3 grams), acrylic acid (2.9 grams) and ethoxylated(9) trimethylolpropane triacrylate (“SR502”) (1.5 gram) were added tothe flask. 2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.15 gram)was added and the contents stirred to dissolve the2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”). The contents of theflask were then purged with N₂ gas for 3 minutes. The sample(translucent and low viscosity) was charged to cylindrical containers(29 mm diameter). Each container was about 18 ml in volume and each wassealed on both ends (very little air gap was left between the top andliquid). The samples were allowed to stand about 12 hr then placed in anoven to cure (50° C., 4 hours). This results in a clear translucent bluegel. The gel was removed from the container and placed in a 473 ml widemouth jar. The jar was filled with ethanol (denatured). The sample wassoaked for 24 hours then the ethanol was replaced with fresh ethanol.The sample was soaked for 24 hours then the ethanol was replaced with athird batch of fresh ethanol. The sample was allowed to soak until thesupercritical extraction was done. The above manipulations were doneminimizing the amount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 29 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 29 was 22.5 grams. About 765 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 29 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 29 aerogel was semi-translucent with a bluishtint and weighed 11.9 grams, corresponding to an overall weight lossduring the supercritical extraction process of 47.1%.

Example 30

To prepare Example 30, Sol S3 was diafiltered and concentrated asdescribed above for Sol T1. The resulting sol was 54.2 wt. % ZrO₂/Y₂O₃and 5.6 wt. % acetic acid. The sol (50 grams) was charged to 500 ml RBflask: Ethanol (15.15 grams), acrylic acid (2.9 grams) and ethoxylated(15) trimethylolpropane triacrylate (“SR9035”) (1.5 gram) were added tothe flask. 2,2′-azobis(2-methylbutyronitrile) (“VAZO 67”) (0.15 gram)was added and the contents stirred to dissolve the2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”). The contents of theflask were then purged with N₂ gas for 3 minutes. The sample(translucent and low viscosity) was charged to cylindrical containers(29 mm diameter). Each container was about 18 ml in volume and each wassealed on both ends (very little air gap was left between the top andliquid). The samples were allowed to stand about 12 hours then placed inan oven to cure (50° C., 4 hours). This results in a clear translucentblue gel. The gel was removed from the container and placed in a 473 mlwide mouth jar. The jar was filled with ethanol (denatured). The samplewas soaked for 24 hours then the ethanol was replaced with freshethanol. The sample was soaked for 24 hours then the ethanol wasreplaced with a third batch of fresh ethanol. The sample was allowed tosoak until the supercritical extraction was done. The abovemanipulations were done minimizing the amount of time the gel wasexposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 30 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 30 was 22.1 grams. About 765 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 30 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 30 aerogel was semi-translucent with a bluishtint and weighed 11.9 grams, corresponding to an overall weight lossduring the supercritical extraction process of 46.2%.

Example 31

For Example 31, 92.36 grams of diafiltered and concentrated Sol C3 (29.5wt. % oxide and 3.1 wt. % acetic acid) was charged to 500 ml RB flask.Water (42.4 grams) was removed via rotary evaporation resulting in aviscous somewhat dry material. Ethanol (15.2 grams), acrylic acid (2.9grams), ethoxylated pentaerythritol tetraacrylate (“SR454”) (1.5 gram)were added to the flask. The contents were stirred about 2 daysresulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.15 gram) was addedand stirred until dissolved. The contents of the flask were then purgedwith N₂ gas for 3 minutes. The sample (translucent and low viscosity)was charged to cylindrical containers (29 mm diameter). Each containerwas about 18 ml in volume and each was sealed on both ends (very littleair gap was left between the top and liquid). The samples were allowedto stand for about 1 hour then placed in an oven to cure (50° C., 4hours). This results in a clear translucent blue gel. The gel wasremoved from the container and placed in a 473 ml wide mouth jar. Thejar was filled with ethanol (denatured). The sample was soaked for 24hours then the ethanol was replaced with fresh ethanol. The sample wassoaked for 24 hours then the ethanol was replaced with a third batch offresh ethanol. The sample was allowed to soak until the supercriticalextraction was done. The above manipulations were done minimizing theamount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 31 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 31 was 21.7 grams. About 790 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 31 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 31 aerogel was semi-translucent with a bluishtint.

Example 32

For Example 32, 92.4 grams of diafiltered and concentrated Sol C3 (29.5wt. % oxide and 3.1 wt. % acetic acid) was charged to a 500 ml RB flask.Water (42.3 grams) was removed via rotary evaporation resulting in aviscous somewhat dry material. Ethanol (15.2 grams), acrylic acid (2.9grams), ethoxylated (15) trimethylolpropane triacrylate (“SR9035”) (1.5gram) were added to the flask. The contents were stirred about 2 daysresulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile), (VAZO 67”) (0.15 gram) was added andstirred until dissolved. The contents of the flask were then purged withN₂ gas for 3 minutes. The sample (translucent and low viscosity) Wascharged to cylindrical containers (29 mm diameter). Each container wasabout 18 ml in volume and each was sealed on both ends (very little airgap was left between the top and liquid). The samples were allowed tostand for about 1 hour then placed in an oven to cure (50° C., 4 hours).This results in a clear translucent blue gel. The gel was removed fromthe container and placed in a 473 ml wide mouth jar. The jar was filledwith ethanol (denatured). The sample was soaked for 24 hours then theethanol was replaced with fresh ethanol. The sample was soaked for 24hours then the ethanol was replaced with a third batch of fresh ethanol.The sample was allowed to soak until the supercritical extraction wasdone. The above manipulations were done minimizing the amount of timethe gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 32 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 32 was 20.4 grams. About 790 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 32 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 32 aerogel was semi-translucent with a bluishtint.

Example 33

For Example 33, 83.1 grams of diafiltered and concentrated Sol C3 (29.5wt. % oxide and 3.1 wt. % acetic acid) was charged to a 500 ml RB flask.Water (42.5 grams) was removed via rotary evaporation resulting in aviscous somewhat dry material. Ethanol (15.15 grams), acrylic acid (2.9grams), butylacrylate (1.5 gram) were added to the flask. The contentswere stirred overnight resulting in a fluid translucent sol.2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.15 gram) was addedand stirred until dissolved. The contents of the flask were then purgedwith N2 gas for 3 minutes. The sample (translucent and low viscosity)was charged to cylindrical containers (29 mm diameter). Each containerwas about 18 ml in volume and each was sealed on both ends (very littleair gap was left between the top and liquid). The samples were allowedto stand for about 1 hour then placed in an oven to cure (50° C., 4hours). This results in a clear translucent blue gel. The gel wasremoved from the container and placed in a 473 ml wide mouth jar. Thejar was filled with ethanol (denatured). The sample was soaked for 24hours then the ethanol was replaced with fresh ethanol. The sample wassoaked for 24 hours then the ethanol was replaced with a third batch offresh ethanol. The sample was allowed to soak until the supercriticalextraction was done. The above manipulations Were done minimizing theamount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 33 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 33 was 21.2 grams. About 765 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 33 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 33 aerogel was semi-translucent with a bluishtint and weighed 11.9 grams, corresponding to an overall weight lossduring the supercritical extraction process of 43.9%.

Example 34

For Example 34, 117.9 grams of diafiltered and concentrated Sol C2 (23.1wt. % oxide and 2.4 wt. % acetic acid) was charged to a 500 ml RB flask.Water (67.9 grams) was removed via rotary evaporation resulting in aviscous somewhat dry material. Ethanol (15.2 grams), acrylic acid (4.6grams), HEMA (2.4 grams) and DI water (1.8 gram) were added to theflask. The contents were stirred overnight resulting in a fluidtranslucent sol. 2,2′-azobis(2-methylbutyronitrile), (“VAZO 67”) (0.15gram) was added and stirred until dissolved. The contents of the flaskwere then purged with N2 gas for 3 minutes. The sample (translucent andlow viscosity) was charged to cylindrical containers (29 mm diameter).Each container was about 18 ml in volume and each was sealed on bothends (very little air gap was left between the top and liquid). Thesamples were allowed to stand for about 1 hour then placed in an oven tocure (50° C., 4 hours). This results in a clear translucent blue gel.The gel was removed from the container and placed in a 473 ml wide mouthjar. The jar was filled with ethanol (denatured). The sample was soakedfor 24 hours then the ethanol was replaced with fresh ethanol. Thesample was soaked for 24 hours then the ethanol was replaced with athird batch of fresh ethanol. The sample was allowed to soak until thesupercritical extraction was done. The above manipulations were doneminimizing the amount of time the gel was exposed to the air.

Extraction Process

The wet ZrO₂-based gel of Example 34 was removed from the ethanol bath,weighed, placed inside a small canvas pouch, and then stored briefly inanother ethanol bath before being loaded into the 10-L extractor vessel.The wet weight of Example 34 was 20.6 grams. About 820 ml of 200-proofethanol was added to the 10-L extractor of a laboratory-scalesupercritical fluid extractor unit. The canvas bag containing the wetzirconia-based gel was transferred from the ethanol bath into the 10-Lextractor so that the wet gel was completely immersed in the liquidethanol inside the jacketed extractor vessel, which was heated andmaintained at 60° C. The Example 34 sample was subjected to the sameextraction process as described above for Examples 1 and 2 samples.Afterwards, the dry aerogel was removed from its canvas pouch, weighed,and transferred into a 237 ml glass jar packed with tissue paper forstorage. The dry Example 34 aerogel was semi-translucent with a bluishtint and weighed 11.5 grams, corresponding to an overall weight lossduring the supercritical extraction process of 44.2%.

Foreseeable modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this invention. This invention should not be restricted tothe embodiments that are set forth in this application for illustrativepurposes.

1. An monolithic aerogel comprising organic material and crystallinemetal oxide particles, wherein the crystalline metal oxide particles arein a range from 3 to 20 volume percent, based on the total volume of themonolithic aerogel, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂.
 2. The monolithic aerogel of claim 1, wherein thecrystalline metal oxide particles are in a range from 1 to 15 molepercent of the crystalline metal oxide is Y₂O₃.
 3. The monolithicaerogel of claim 1, wherein the crystalline metal oxide particlescomprise a first plurality of particles, and a second, differentplurality of particles.
 4. A method of making a crack-free, calcinedmetal oxide article having x, y, and z dimensions of at least 5 mm, adensity in as range from 30 to 95 percent of theoretical density, and anaverage connected pore size in a range from 10 nm to 100 nm, wherein atleast 70 mole percent of the metal oxide is crystalline ZrO₂, andwherein the crystalline ZrO₂ has an average grain size less than 100 nm,the method comprising heating the monolithic aerogel of claim 1 for atime and at at least one temperature sufficient to provide thecrack-free, calcined metal oxide article.
 5. The method of claim 4further comprising chemically treating the calcined metal oxide articleto remove volatile ions.
 6. A method of making the aerogel comprisingorganic material and crystalline metal oxide particles, wherein thecrystalline metal oxide particles are in a range from 3 to 20 volumepercent, based on the total volume of the aerogel, wherein at least 70mole percent of the crystalline metal oxide is ZrO₂, the methodcomprising: providing a first zirconia sol comprising crystalline metaloxide particles having an average primary particle size of not greaterthan 50 nanometers, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂; optionally concentrating the first zirconia sol toprovide as concentrated zirconia sol; adding a radically reactivesurface modifier to the zirconia sol to provide a radicallypolymerizable surface-modified zirconia sol; adding a radical initiatorto the radically polymerizable surface-modified zirconia sol; heating atat least one temperature for a time sufficient to polymerize theradically surface-modified zirconia sol comprising the radical initiatorto form a gel; optionally removing water, if present, from the gel viaalcohol exchange to provide an at least partially de-watered gel; andextracting alcohol, if present, from the gel via super criticalextraction to provide the aerogel.
 7. A crack-free, calcined metal oxidearticle having x, y, and z dimensions of at least 5 mm, a density in arange from 30 to 95 percent of theoretical density, and an averageconnected pore size in a range from 10 nm to 100 nm, wherein at least 70mole percent of the metal oxide is crystalline ZrO₂, and wherein thecrystalline ZrO₂ has an average grain size less than 100 nm.
 8. Thecrack-free, calcined metal oxide of claim 7, wherein the crystallinemetal oxide comprises in a range from 1 to 15 mole percent of thecrystalline metal oxide is Y₂O₃.
 9. A method of making a crack-free,crystalline metal oxide article having an x, y, and z dimensions of atleast 3 mm and a density of at least 98.5 percent of theoreticaldensity, wherein at least 70 mole percent of the crystalline metal oxideis ZrO₂, and wherein the ZrO₂ has an average grain size less than 400nanometers, the method comprising heating a crack-free, calcined metaloxide article of claim 6 for a time and at at least one temperaturesufficient to provide the crack-free, crystalline metal oxide article.10. A crack-free, crystalline metal oxide article having an x, y, and zdimensions of at least 3 mm and a density of at least 98.5 percent oftheoretical density, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂, and wherein the ZrO₂ has an average grain size in arange from 75 nanometers to 400 nanometers.
 11. The crack-free,crystalline metal oxide article of claim 10, wherein the crack-free,crystalline metal oxide article is a dental article.
 12. The crack-free,crystalline metal oxide article of claim 10, wherein the crack-free,crystalline metal oxide article is an orthodontic appliance.
 13. Acrack-free, crystalline metal oxide article having an x, y, and zdimensions of at least 3 mm and a density of at least 98.5 percent oftheoretical density, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂, wherein in range from 1 to 5 mole percent of thecrystalline metal oxide is Y₂O₃, and wherein the ZrO₂ has an averagegrain size 75 nanometers to 175 nanometers.
 14. A method of making acrack-free, crystalline metal oxide article of claim 13, the methodcomprising heating a crack-free, calcined metal oxide article for a timeand at at least one temperature sufficient to provide the crack-free,crystalline metal oxide article, the crack-free, calcined metal oxidearticle having an x, y, and z dimensions of at least 3 mm, a density ina range from 30 to 95 percent of theoretical density, and an averageconnected pore size in a range from 10 nm to 100 nm, wherein at least 70mole percent of the metal oxide is crystalline ZrO₂, wherein at least 70mole percent of the crystalline metal oxide is ZrO₂, wherein in rangefrom 1 to 5 mole percent of the crystalline metal oxide is Y₂O₃, andwherein the crystalline ZrO₂ has an average grain size less than 100 nm.15. A crack-free, crystalline metal oxide article having an x, y, and zdimensions of at least 3 mm and a density of at least 98.5 percent oftheoretical density, wherein at least 70 mole percent of the crystallinemetal oxide is ZrO₂, wherein in range from 6 to 9 mole percent of thecrystalline metal oxide is Y₂O₃, and wherein the ZrO₂ has an averagegrain size in a range from 100 nanometers to 400 nanometers.
 16. Amethod of making a crack-free, crystalline metal oxide article of claim15, the method comprising heating a crack-free, calcined metal oxidearticle for a time and at at least one temperature sufficient to providethe crack-free, crystalline metal oxide article, the crack-free,calcined metal oxide article having x, y, and z dimensions of at least 3mm, a density in a range from 30 to 95 percent of theoretical density,and an average connected pore size in a range from 10 nm to 100 nm,wherein at least 70 mole percent of the metal oxide is crystalline ZrO₂,wherein at least 70 mole percent of the crystalline metal oxide is ZrO₂,wherein in range from 6 to 9 mole percent of the crystalline metal oxideis Y₂O₃, and wherein the crystalline ZrO₂ has an average grain size lessthan 100 nm
 17. A method of making a crack-free, crystalline metal oxidearticle having an x, y, and z dimensions of at least 3 mm and a densityof at least 98.5 percent of theoretical density, wherein at least 70mole percent of the crystalline metal oxide is ZrO₂, and wherein theZrO₂ has an average grain size less than 300 nanometers, the methodcomprising pressureless heating in air a crack-free, calcined metaloxide article having x, y, and z dimensions of at least 5 mm, a densityof at least 30 percent of theoretical density, wherein at least 70 molepercent of the metal oxide is crystalline ZrO₂, and wherein thecrystalline ZrO₂ has an average grain size less than 100 nm for a timeand at at least one temperature sufficient to provide the crack-free,crystalline metal oxide article, wherein the method is conducted at nogreater than 1400° C.